Author Topic: Non-Zero Crossing AC SSR: Use with DC?  (Read 6865 times)

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Offline TimNJTopic starter

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Non-Zero Crossing AC SSR: Use with DC?
« on: October 20, 2019, 02:28:43 am »
Hi all,

I'm looking for a relay to use in an SMPS inrush tester I am designing. After pushing a button (or driving the input with a square wave signal), the circuit fires the relay at the peak of the next AC cycle. The circuit uses a zero-crossing detector and then applies a delay to turn the relay on exactly at the peak of the AC wave.

I was planning on using an solid state relay (SSR) because I think it should have cleaner turn on/off characteristics. It's important that there isn't a crazy amount of ringing on the measurement.

Many SSRs are zero-crossing-switching, meaning they will only turn on or off at a zero-crossing. But, many manufacturers will also offer an instantaneous-switching version. Here is one I am considering: http://www.crydom.com/en/products/pcb-mount/ac-output/pf-series/

Now, if I want to use this same tester to switch DC, is it possible? From my research, most people say AC SSRs cannot be used with DC because there is no zero-crossing. However, if I use an instantaneous-switching SSR, is this still an issue? Any other issues?

Thanks!
Tim

P.S. Current schematic attached, for reference.


 

Offline MagicSmoker

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #1 on: October 20, 2019, 09:56:40 am »
...
Now, if I want to use this same tester to switch DC, is it possible? From my research, most people say AC SSRs cannot be used with DC because there is no zero-crossing. However, if I use an instantaneous-switching SSR, is this still an issue? Any other issues?
...

If the SSR uses a triac or back-to-back SCRs you can't use it on DC unless you are fine with cycling the incoming power to turn it off (or load current drops below the holding current of the thyristor(s)). If the SSR uses MOSFETs for the output switch then you can use it on AC or DC (note that for AC there needs to be two MOSFETs wired in inverse series - that is, source to source).
 
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Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #2 on: October 20, 2019, 02:04:02 pm »
...
Now, if I want to use this same tester to switch DC, is it possible? From my research, most people say AC SSRs cannot be used with DC because there is no zero-crossing. However, if I use an instantaneous-switching SSR, is this still an issue? Any other issues?
...

If the SSR uses a triac or back-to-back SCRs you can't use it on DC unless you are fine with cycling the incoming power to turn it off (or load current drops below the holding current of the thyristor(s)). If the SSR uses MOSFETs for the output switch then you can use it on AC or DC (note that for AC there needs to be two MOSFETs wired in inverse series - that is, source to source).

Ah. Thanks. I knew they had back-to-back SCRs but I don't really understand SCRs very much, so that limitation didn't really make sense to me.

On that note, another thing I noticed is the relatively high minimum/holding current rating for these devices. For the Crydom PF, it says 60mA minimum. I'd like to make sure the relay stays on even if the SMPS is in a light load condition. To satisfy 60mA @ 90VAC, need about a 1.5K resistor. But, since I'd like this to also be compatible with high AC line voltage (264V), then the dissipation of the 1.5K resistor @ 264V will be about 50W. Yikes!

It's only a test fixture, so maybe it's okay, but seems a bit crazy.

Thanks again.
 

Offline MagicSmoker

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #3 on: October 20, 2019, 02:24:26 pm »
...
On that note, another thing I noticed is the relatively high minimum/holding current rating for these devices. For the Crydom PF, it says 60mA minimum. I'd like to make sure the relay stays on even if the SMPS is in a light load condition. To satisfy 60mA @ 90VAC, need about a 1.5K resistor. But, since I'd like this to also be compatible with high AC line voltage (264V), then the dissipation of the 1.5K resistor @ 264V will be about 50W. Yikes!
...

There's a couple of solutions to the minimum holding current issue that don't require wasting any heat in an external minimum load resistance. The first is to wire a decent quality film capacitor rated for use across the mains in parallel with the SMPS-under-test; 2.2uF should do the trick. The second is to simply provide a continuous trigger signal to the SSR for the entire time you need the load to be on.  ;)


EDIT - added the wink at the end.
« Last Edit: October 20, 2019, 07:51:06 pm by MagicSmoker »
 
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Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #4 on: October 20, 2019, 03:16:48 pm »
Oh...well that's interesting. Do I not understand how an SSRs is typically triggered? My plan was to provide continual 5V or 12V to the "coil" terminals just like a normal non-latching relay.

Regarding the capacitor, I thought of that, but thought I wouldn't want it because it would throw off the inrush measurement. However, I realize now that you can avoid measurement of the X-cap charging by simply measuring the current on the output of the tester, instead of on the input side. |O

Thanks once again.
 

Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #5 on: October 20, 2019, 04:23:05 pm »
Some more discoveries for anyone playing along at home.

After reading a few Wikipedia pages for about 10 minutes  :clap:,  I learned the following:

1. An SSR (using SCRs) can be turned on quite easily (and at any point in the AC cycle) by simply applying a gate voltage.
2. An SSR (using SCRs) will inherently turn off when the current drops to 0 after the gate signal is removed.

This is why the random/instantaneous/non-ZCD SSRs have a turn on spec of about 0.1ms, but a turn off spec of up to 1/2cycle (max). You cannot turn off the SCR at an arbitrary point in the AC line, and more specifically at any point on the current waveform.

That's a good thing for me, since I can delete my ZCD shut off circuit.

However, I wonder if that will affect whether my 50/60Hz SSR will shut off with a 400Hz source. I guess I'll have to check..

Thanks.
 

Offline MagicSmoker

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #6 on: October 20, 2019, 07:50:38 pm »
Oh...well that's interesting. Do I not understand how an SSRs is typically triggered? My plan was to provide continual 5V or 12V to the "coil" terminals just like a normal non-latching relay.

No, I just forgot to put the ;) at then end of my earlier reply because you normally provide the trigger signal for as long as you the load to be on with an SSR.  :palm:

As for operation at 400 Hz, you may want to check with the manufacturer because SCRs - but especially triacs - are not known for being especially fast. You also need to be very careful to not exceed the max dI/dt spec during turn on, and this may already be an issue given the type of test you are trying to perform here (ie - inrush measurement).
 
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Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #7 on: October 21, 2019, 03:39:02 pm »
Thanks again.

Honestly, I think the joke went right over my head. (I think I still don't quite get what you are saying.) Does holding the trigger help to keep the SCRs on or not?

Regarding 400Hz operation, I understand your points.

The I2t rating for the Crydom PF is 260A2/sec, rated at 8.3msec. Sounds like that will be fine. I2t = 1/2 *(Ip)2*t, where Ip is the peak current and t is the width of the half sine pulse.

Based on the 260A2/sec rating, I believe that means the peak current can be 245A at 60Hz. That said, there are typically a few subsequent half-sine pulse, but I guess we can (sort of) disregard those. Factor in a little extra margin to account for them. The switch mode power supplies I deal with usually have inrush currents <100A.

 

Offline MagicSmoker

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #8 on: October 21, 2019, 04:07:08 pm »
Honestly, I think the joke went right over my head. (I think I still don't quite get what you are saying.) Does holding the trigger help to keep the SCRs on or not?

For the purpose of this discussion, if the supply is AC then then a triac or back-to-back wired SCRs will turn off every zero crossing unless continuously triggered. If continuously triggered then there will still be a brief period of non-conduction around every zero crossing (so some distortion of the current waveform) when the voltage is insufficient to overcome the forward drop (typically given as Vtm in the datasheet).


The I2t rating for the Crydom PF is 260A2/sec, rated at 8.3msec....

Nope, that is the fusing energy rating - basically, if you want a fuse to protect the triac/scr from overload it needs to have a lower I2t rating than the triac. The spec you are interested in is the max dI/dt rating... which apparently isn't given in that datasheet, I see. So, again, better confirm with the manufacturer. FWIW, back-to-back SCRs are superior to triacs when it comes to dI/dt, dV/dt and I2t at the cost of being more difficult to trigger (there isn't a common reference for the trigger signal to each SCR).

 
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Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #9 on: October 21, 2019, 05:23:50 pm »
The I2t rating for the Crydom PF is 260A2/sec, rated at 8.3msec....

Nope, that is the fusing energy rating - basically, if you want a fuse to protect the triac/scr from overload it needs to have a lower I2t rating than the triac. The spec you are interested in is the max dI/dt rating... which apparently isn't given in that datasheet, I see. So, again, better confirm with the manufacturer. FWIW, back-to-back SCRs are superior to triacs when it comes to dI/dt, dV/dt and I2t at the cost of being more difficult to trigger (there isn't a common reference for the trigger signal to each SCR).

I see. I thought "fusing" energy, in this context, was the point at which the SSR "fuses" (aka fails), kind of like how fusible resistors are specified. Maybe that's still true, but I see that the fusing energy is still separate idea from the dI/dt rating. Let's see if Crydom will talk to me.  ;)

Thanks for your help.
 

Offline james_s

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #10 on: October 21, 2019, 07:59:58 pm »
The zero crossing switching offered by some SSRs refers to the turn-on point, a zero cross SSR will always turn on at the zero cross. All AC SSRs will turn off at the zero cross, this is part of the standard behavior of a thyristor. Once triggered they latch on until the current through them falls below a minimum threshold.
 
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Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #11 on: October 22, 2019, 02:43:29 am »
The zero crossing switching offered by some SSRs refers to the turn-on point, a zero cross SSR will always turn on at the zero cross. All AC SSRs will turn off at the zero cross, this is part of the standard behavior of a thyristor. Once triggered they latch on until the current through them falls below a minimum threshold.

Thanks. Yes, I just learned (re-learned?) that. Never worked on a project with SCRs before so didn't have that engrained in my memory yet!

I think I'm getting the itch to feature-creep in a DIY MOSFET based solid state relay. Seems like a MOSFET based relay would be superior in this application. Plus it'll work with DC.

Why are the high power MOSFET based SSR options so limited? Is there something I'm missing? Why are SCR based relays still king? Inherent zero current switching?
 

Offline james_s

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #12 on: October 22, 2019, 03:28:29 am »
SCRs (and triacs) are inexpensive, robust, easy to trigger, and they have a low forward drop so the dissipation is reasonable.

SSR's are typically used to control AC line powered loads from low voltage DC control systems. When one needs to control a largish DC load it's usually easier to just use low side switching with a single transistor or use a mechanical relay. The advantages offered by a SSR aren't really needed.
 
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Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #13 on: October 22, 2019, 04:22:16 am »
SCRs (and triacs) are inexpensive, robust, easy to trigger, and they have a low forward drop so the dissipation is reasonable.

SSR's are typically used to control AC line powered loads from low voltage DC control systems. When one needs to control a largish DC load it's usually easier to just use low side switching with a single transistor or use a mechanical relay. The advantages offered by a SSR aren't really needed.

Thanks. I see that the price of 2 MOSFETs with comparable combined voltage drop (at rated current) is about $10-15USD, whereas the cost of the Crydom PF module is about $30. Crydom part is obviously very thoroughly tested and easy to use, without much fuss. Need to add in isolated DC-DC converter, optocoupler, etc. for MOSFET version, so in the end, it seems like the price might come out about the same.

I can see how the latching action of an SCR could be advantageous in some applications.

In terms of robustness, are you just saying they are "generally" robust? It seems like MOSFETs are also pretty robust these days.
« Last Edit: October 22, 2019, 04:26:36 am by TimNJ »
 

Offline Circlotron

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #14 on: October 22, 2019, 04:25:23 am »
On that note, another thing I noticed is the relatively high minimum/holding current rating for these devices. For the Crydom PF, it says 60mA minimum. I'd like to make sure the relay stays on even if the SMPS is in a light load condition. To satisfy 60mA @ 90VAC, need about a 1.5K resistor. But, since I'd like this to also be compatible with high AC line voltage (264V), then the dissipation of the 1.5K resistor @ 264V will be about 50W. Yikes!
You could always put a normal mechanical relay across the SSR and trigger them both at the same instant. The SSR will turn on almost immediately and the mechanical relay with its contacts across the SSR will follow several milliseconds later.
 

Offline beduino

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #15 on: October 22, 2019, 07:45:06 am »
I think I'm getting the itch to feature-creep in a DIY MOSFET based solid state relay. Seems like a MOSFET based relay would be superior in this application. Plus it'll work with DC.

A few AC mosftes switches in parallel should do the job depending on how low resistance needed and voltage levels ;)

 
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Offline Zero999

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #16 on: October 22, 2019, 12:15:15 pm »
Now, if I want to use this same tester to switch DC, is it possible?
Why not design a separate tester for DC? That would be much easier, rather than something which will need to work with both AC and DC.

If it doesn't need to turn off, a simple SCR will do. Once triggered it will turn on and not switch off again, until the power is interrupted.

It does need to turn off, a MOSFET can be used.

Isolation can be provided with an opto-TRIAC for the SCR and an opto-photovoltaic cell for the MOSFET.
 
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Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #17 on: October 22, 2019, 01:30:14 pm »
Now, if I want to use this same tester to switch DC, is it possible?
Why not design a separate tester for DC? That would be much easier, rather than something which will need to work with both AC and DC.

If it doesn't need to turn off, a simple SCR will do. Once triggered it will turn on and not switch off again, until the power is interrupted.

It does need to turn off, a MOSFET can be used.

Isolation can be provided with an opto-TRIAC for the SCR and an opto-photovoltaic cell for the MOSFET.

I agree with you that often times it's much better to make a single purpose device instead of making something that "does it all". However, if it's not too much extra work, it might be less work to tack on DC function to the AC tester. However, as I think about it more, I'll probably wind up making two separate testers because I might want to put some chunky caps on the DC input to get the source impedance down as low as possible.

I think I'll try designing a MOSFET SSR first (as a standalone module) and then integrate that into the full tester. There are some cool opto-PV gate drivers like Vishay's VOM1271 which seems reasonable. I don't really think switching speed is that big of a deal in this case, but they still are pretty slow...I'm thinking a little 1W isolated DC-DC converter + regular opto-coupler might have better performance without being overly complex or expensive.

Thanks!

 

Offline MagicSmoker

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #18 on: October 22, 2019, 02:16:39 pm »
...
I don't really think switching speed is that big of a deal in this case, but they still are pretty slow...I'm thinking a little 1W isolated DC-DC converter + regular opto-coupler might have better performance without being overly complex or expensive.

The last sentence outlines the best approach - go with inverse series connected MOSFETs driven by a proper gate driver and a small isolated dc-dc converter to supply it because you do need decently fast switching speeds given that you are trying to maximize inrush current for testing purposes. You really don't want the MOSFETs taking >1ms to turn on while also trying to conduct a huge peak current.

 

Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #19 on: October 28, 2019, 01:00:49 am »
As I'm work on this MOSFET SSR design, I am beginning to understand why SCR based SSRs tend to dominate in the high power segment.

For one, zero-crossing switching, a freebie for SCRs, has to be added to a MOSFET design. Some preliminary modeling shows that switching off a 1KW load with 1 or 2mH of inductance will produce crazy ringing with 1-2KV peaks! That's not surprising, but still a problem.

My best solution so far is a current transformer + comparator which detects when the current is at or near zero. But of course, that makes the relay physically bigger extra and adds a few dollars to the BOM..

I'm curious: Is there any "real" application where a MOSFET based (high power) SSR would be advantageous? Any application where you need to control the turn on of something even faster, or with greater precision?

Thanks.
 

Offline MagicSmoker

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #20 on: October 28, 2019, 07:26:07 pm »
...
I'm curious: Is there any "real" application where a MOSFET based (high power) SSR would be advantageous? Any application where you need to control the turn on of something even faster, or with greater precision?

Most MOSFET SSRs switch much more quickly than any thyristor type; the possible exception is when a photovoltaic isolator is used (literally an LED shining on a PV stack that generates a feeble current at enough voltage to turn on the MOSFET... usually over a period of hundreds of microseconds).

Insufficiently snubbed thyristor SSRs may fail to turn off at the next zero crossing with very low power factor loads whereas AC/DC MOSFET SSRs retain positive control of turn-off for DC loads, so won't care about the power factor of an AC load.

That said, thyristors are far more rugged than MOSFETs for a given die area and nominal current rating. A 20A thyristor will happily switch tungsten lamps on and off all day long which can demand up to 10x nominal current during inrush, whereas a MOSFET will likely need to be much more conservatively spec'ed; perhaps needing a 100A-rated device for the same application.

Rarely is there a one-size-fits-all answer, as seems to always be the case in engineering.
 
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Offline SeanB

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #21 on: October 28, 2019, 07:57:11 pm »
Yes, the first DC power switch semiconductor was the SCR, though there you always had at least 2 of them, some really high value inductors, high power resistors and big capacitors to allow one SCR to be used to force commutate the other. But you could have a 200A SCR controlling a DC motor at around 100Hz PWM rate to allow variable speed control, which was a massive jump in efficiency over the previous method, which involved both multiple tappings off the battery bank and some serious power resistors to give stepped control. SCR you could use a single battery voltage, have control from near 0% to 100% with near stepless control, along with feedback so your motor speed was constant despite massive load variations. High voltage withstand when off, high current when on, and pretty much almost no switching loss in operation until you got into the audible frequency ranges, still the preferred device for ultra high power use, where IGBT devices are only now starting to get serious attention.
 

Offline Circlotron

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #22 on: October 28, 2019, 09:02:33 pm »
Yep, force commutated SCRs are a discipline all their own. About as sophisticated as a steam locomotive, but also just as reliable. Huge overload capacity, as has been mentioned. Lots of interesting resonant circuits involved, and really getting to grips with basic fundamentals. Kinda sad in a way that they are gone from general use.
 

Offline TimNJTopic starter

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #23 on: October 28, 2019, 10:10:16 pm »
...
I'm curious: Is there any "real" application where a MOSFET based (high power) SSR would be advantageous? Any application where you need to control the turn on of something even faster, or with greater precision?

Most MOSFET SSRs switch much more quickly than any thyristor type; the possible exception is when a photovoltaic isolator is used (literally an LED shining on a PV stack that generates a feeble current at enough voltage to turn on the MOSFET... usually over a period of hundreds of microseconds).

Insufficiently snubbed thyristor SSRs may fail to turn off at the next zero crossing with very low power factor loads whereas AC/DC MOSFET SSRs retain positive control of turn-off for DC loads, so won't care about the power factor of an AC load.

That said, thyristors are far more rugged than MOSFETs for a given die area and nominal current rating. A 20A thyristor will happily switch tungsten lamps on and off all day long which can demand up to 10x nominal current during inrush, whereas a MOSFET will likely need to be much more conservatively spec'ed; perhaps needing a 100A-rated device for the same application.

Rarely is there a one-size-fits-all answer, as seems to always be the case in engineering.

Thanks for all of that information. It's clear I'm lacking in my knowledge of thyristors, and I definitely want to learn more.

I guess my question still remains. What might be some applications where you need to turn on a big AC load in the matter of microseconds? It seems like most (industrial?) loads of that nature will probably be fine to turn on within a few 10s of milliseconds.

I'm just confused because before it seemed like people were suggesting that an SCR based solution (for the inrush tester) would NOT be as robust as a MOSFET solution. But maybe I misconstrued that idea. MagicSmoker suggested that I might exceed the dI/dt rating of an SCR SSR if used for inrush testing. Are MOSFETs any better at that?

To me, it seems like I should just use a beefy SCR at this point. Or maybe even drop the whole DIY SSR concept completley and use an off the shelf SSR as originally planned.
 

Offline Circlotron

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Re: Non-Zero Crossing AC SSR: Use with DC?
« Reply #24 on: October 28, 2019, 10:41:33 pm »
I guess my question still remains. What might be some applications where you need to turn on a big AC load in the matter of microseconds? It seems like most (industrial?) loads of that nature will probably be fine to turn on within a few 10s of milliseconds.
It's not so much that the load needs to be turned on in microseconds but the switching device, mosfet, SCR etc needs to fully turn on quickly. When it is transitioning from off to on, depending on the nature of the load, it can be dissipating a LOT of power internally. Best to get through that transition as quick as possible.
 
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