Modify Voltage of SMPS Brick

[0xPIT]

Hello,

I currently fail modding a LiteOn SMPS from 12V to 24V, though having modified countless other SMPSs successfully before.

The relevant part of the circuit is attached to this post. (unpopulated parts removed)

My approach was to modify the resistor divider formed by R62 + R64 (R65 = 0 Ohm).
It's resulting voltage is compared with the internal 2.5V reference voltage.

As mere calculation of the divider did not result in a working PSU, I've replaced R64 with a pot...
Whenever the output voltage exceeds about 19V DC, the PSU shuts down – I fail to understand why.
Can anyone give a hint?

Note that I've also replaced R57 by double the value in order to not blow the LED in the OC, and C54 with a 25V type.

Thanks!

[glarsson]

A quick guess:
Output on pin 1 is voltage feedback via the optocoupler.
Output on pin 7 is over voltage protection via the optocoupler.
D11 acts as an OR-gate.
Resistors R68, R69 and R70 probably sets the over voltage point.
When output reaches 19V output 7 triggers a shutdown.

[Armadillo]

Countless ! LOL 

By that token and your reasoning, making the divider variable, you can mod? into a benchtop power supply! ?   

Obviously one switching transformer cannot be taken to suit all voltages, it went into saturation and the inductance factor drops drastically, and the transistor went into pulling high amp and you are fortunate the management IC swung into protection mode quick enough otherwise it will bang & boom.

[0xPIT]

Output on pin 7 is over voltage protection via the optocoupler.

No, this is over-current protection, as it's after R50 (which is 0.8R)

D11 acts as an OR-gate.
Resistors R68, R69 and R70 probably sets the over voltage point.
When output reaches 19V output 7 triggers a shutdown.

D11 is an AND gate.
R68 is just Bias for the Reference, R69+70 are the voltage divider for the over-current comparator. (2.5Vref -> set to 0.33V)

Also, I've lifted R60 to make sure over-current won't hit.

[0xPIT]

By that token and your reasoning, making the divider variable, you can mod? a benchtop power supply! ?   

Yes, works with most fixed-voltage SMPSs, typically 3.3 to ~30V is not a problem at all. I've modified dozens so far.
I assume these devices are designed with that in mind, so they can cheaply produce 5/9/12/24V/etc. models by changing resistors only.

Obviously [switching] transformer [went] into saturation and the inductance factor drops drastically, and the transistor went into pulling high [current] and [the] management IC swung into protection mode [...]

Quite possible. Would be the first PSU in really many to do that, though.
Evidence for your case is that LiteOn seems to offer this particular supply only in 12V configuration.

Hm, I wonder how I can prove your hypothesis.

[xavier60]

There is also likely to be a problem on the live side. Most control ICs monitor the voltage applied to the VCC pin for Over Voltage Protection. The higher output voltage will add to the PIV of the output diodes and to the peak voltage on the Drain of the likely present MOSFET connected to the primary winding. Saturation is not an issue.

[Armadillo]

By that token and your reasoning, making the divider variable, you can mod? a benchtop power supply! ?   

Yes, works with most fixed-voltage SMPSs, typically 3.3 to ~30V is not a problem at all. I've modified dozens so far.
I assume these devices are designed with that in mind, so they can cheaply produce 5/9/12/24V/etc. models by changing resistors only.

Obviously [switching] transformer [went] into saturation and the inductance factor drops drastically, and the transistor went into pulling high [current] and [the] management IC swung into protection mode [...]

Quite possible. Would be the first PSU in really many to do that, though.
Evidence for your case is that LiteOn seems to offer this particular supply only in 12V configuration.

Hm, I wonder how I can prove your hypothesis.

Wow! those are amazing!.   

[0xPIT]

That makes total sense, xavier60!
I can't find a datasheet for the controller IC (LTA809FA). Do you think just doubling the OVP resistor can work?

nvm, found it: http://www.datasheetcatalog.com/datasheets_pdf/S/G/6/7/SG6741.shtml

[xavier60]

Z1 should be doubled. It looks like 20v, another 20v zenner could be added.  allowing pin 7 VDD voltage to double is certain to cause problems. A 3 pin regulator IC could be add. Pick voltage close to the unmodified voltage on pin 7.
The regulator is likely to interfere with startup.  Normally C3 supplies enough energy to power the IC for 50ms or more after it is charged by current drawn by the HV pin.
There are likely to be other surprise problems.

[0xPIT]

Doubling is probably too much, as the IC accepts <= 30V, assuming both transformer-derived voltages are similar.

It seems Z1 forms an additional over voltage protection by disabling the oscillator.
Can't figure out what benefit this has as there's OVP built in the chip, also the reference design does not include that protection.

[xavier60]

Because the IC has a wide operating voltage range, simply placing a zenner in series with D2 should do the trick.

[blueskull]

This is just plain stupidity. Why do you think they put a zener diode to detect axu winding over voltage? When you hack Z1, you are defeating its coil OVP feature, and literally the primary side MOSFET can blow up, and secondary rectifier diodes can blow up.
I don't mind modding voltage to, say, +-10% or even +-20%, but +100% is just insane. You'd have better luck replacing the xfmr with a proper one, or use a voltage doubler rectifier on secondary side.

[xavier60]

I'm expecting the MOSFET's peak drain voltage to rise by about 150v.

[0xPIT]

Why do you think they put a zener diode to detect axu winding over voltage? When you hack Z1, you are defeating its coil OVP feature

With my limited knowledge of power electronics, I don't understand how this is the case.

Even if the Zener would be increased, the OVP of the IC would kick in and stop the PWM.
Both OVP implementations (On-Chip & Zener+Transistor) are sourced from the same coil.

I don't understand why they've implemented a 2nd protection.

As your profile claims specialist knowledge, I'd be grateful for your wisdom.

[capt bullshot]

I don't understand why they've implemented a 2nd protection.

That's for redundancy. If the (for example) optocoupler fails (this is a single point of failure), the controller goes out of regulation, output voltage rises. The zener / transistor OVP prevents further damage then.

[blueskull]

Even if the Zener would be increased, the OVP of the IC would kick in and stop the PWM.
Both OVP implementations (On-Chip & Zener+Transistor) are sourced from the same coil.

I don't understand why they've implemented a 2nd protection.

The internal protection trips at 26V, while the external protection trips at 20V. Depending on transformer ratio (say, n=n_pri:n_aux), then Vdrain=Vaux*n+Vbus+Vringing.

When you increase output voltage without changing transformer, you increase Vdrain, and the extra voltage may cause avalanche breakdown of Q1, and if the avalanche power is higher than thermal dissipation limit of Q1, it blows up.

If you want to play on the stupid side, you can series connect a zener diode to D2. God bless you don't set your house on fire.

[Zucca]

You can series connect a zener diode to D2. God bless you don't set your house on fire.

Do it and then increase the Vmain slowly with a variable trafo watching out if fireworks happen.
Do you have one? If you are modding a lot of SMPS I suggest you to buy one.

EDIT: also a bulb lamp in series between the SMPS and the mains can help.

[capt bullshot]

Yes, works with most fixed-voltage SMPSs, typically 3.3 to ~30V is not a problem at all. I've modified dozens so far.

I've walked a hundred times across the street without watching the traffic, so there's no problem at all doing so.

I assume these devices are designed with that in mind, so they can cheaply produce 5/9/12/24V/etc. models by changing resistors only.

I believe the streets were designed with me walking carelessly across them in mind, so I'm safe.

[0xPIT]

Do it and then increase the Vmain slowly with a variable trafo watching out if fireworks happen.
Do you have one? If you are modding a lot of SMPS I suggest you to buy one.

Thanks, yes I have a variable isolation transformer.

[Armadillo]

Yes, works with most fixed-voltage SMPSs, typically 3.3 to ~30V is not a problem at all. I've modified dozens so far.

I've walked a hundred times across the street without watching the traffic, so there's no problem at all doing so.

I assume these devices are designed with that in mind, so they can cheaply produce 5/9/12/24V/etc. models by changing resistors only.

I believe the streets were designed with me walking carelessly across them in mind, so I'm safe.

Well said !   

[0xPIT]

Ok, thanks a lot, all, I've learned stuff and also came to the conclusion that it's not worth modifying, as this model is not designed for voltages far from 12V.

[0xPIT]

Yes, works with most fixed-voltage SMPSs, typically 3.3 to ~30V is not a problem at all. I've modified dozens so far.

I've walked a hundred times across the street without watching the traffic, so there's no problem at all doing so.

I assume these devices are designed with that in mind, so they can cheaply produce 5/9/12/24V/etc. models by changing resistors only.

I believe the streets were designed with me walking carelessly across them in mind, so I'm safe.

Well said !   

Indeed well said and true, but unfortunately in course of a forum discussion completely useless for anything but the poster's ego.

[Armadillo]

Yes, works with most fixed-voltage SMPSs, typically 3.3 to ~30V is not a problem at all. I've modified dozens so far.

I've walked a hundred times across the street without watching the traffic, so there's no problem at all doing so.

I assume these devices are designed with that in mind, so they can cheaply produce 5/9/12/24V/etc. models by changing resistors only.

I believe the streets were designed with me walking carelessly across them in mind, so I'm safe.

Well said !   

Indeed well said and true, but unfortunately in course of a forum discussion completely useless for anything but the poster's ego.

Pride and the eagerness to show had inhibited the ability to "think" despite being directed and the lax of real intention to learn which defeated the purpose of asking.

Because this is of safety concern, the adamant insistent of the mob may be hazardous to other young readers hence,  the need to be poke and corrected to proper direction.

The lesson is not to repeat such "stupidity". [and which you have indirectly and silently accepted in earlier posting].

Cheers!.   

[IanMacdonald]

You can modify the output of a flyback converter like this over quite a wide range so long as you don't go too far above its (12v) rating.  Push-pull circuits have a more limited output range and are not generally suited to variable supplies.

Main point is that raising the output also raises the reverse (flyback) voltage on the input winding. (Think, transformer effect) Eventually you will reach the point where the input switching transistor pops its clogs.  So, 14v for charging lead acid (for example) would usually be OK, 24v is definitely not.