Electronics > Beginners
how does blackdog's PSU work?
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David Hess:

--- Quote from: Cerebus on March 22, 2018, 03:00:44 pm ---
--- Quote from: exe on March 22, 2018, 10:00:52 am ---Topics such as frequency compensation or choosing the pass element are quite rarely discussed (at least I didn't find much info, yet alone measurements).
--- End quote ---

Don't forget that a power supply is just - in essence - a DC coupled power amplifier. So, texts covering power amplifiers go over much of the same ground. If you want a discussion of pass element choice that's a place to look for it. Just make sure you stick to reliable sources, otherwise you'll find yourself going down the audiophool rabbit hole.
--- End quote ---

This is especially the case because common voltage feedback operational amplifiers duplicate the topology used in audio power amplifiers with a low voltage gain differential input stage, high voltage gain level shifter, and unity gain push-pull output stage.  Remove the upper or lower half of the output stage and a DC coupled audio power amplifier becomes a regulated power supply although some power supplies keep the other half of the output stage for better control or 2 quadrant operation.

The transistors blackdog used in his design are ring or distributed emitter transistors with low Ft droop and hfe droop intended for audio power amplifiers and work great for high performance regulators.  Similar old but now inexpensive parts include the TO-220 D44/D45 transistors which were sometimes found in high performance point of load linear regulators for CPUs and low noise voltage regulator designs.
blackdog:
Hi,

It is true what David says about the transistors that I used.
And I also tested with the D45H, and that went not wel, kaboem!  :-DD

I tested with 4x D45H, this transistor is useful with a 30V 2,5-Ampere power supply, but only if a pre-regulator is used or say transformer tabs to lower the dissipation.
The SOA characteristic is not so good of these transistors, in the Motorola datasheet you can see what the SOA is at 70C.
At 30V Vce max 0.6-Ampere..., so you need a good heatsink.
My preference is for the transistors that are in the schematic.

Kind regards,
Bram
David Hess:
All of the usual warning about SOA (safe operating area) apply to the D44H/D45H series just like any other transistor.  In addition, the high Ft and hfe of ring emitter parts makes them more susceptible to destructive local oscillations if care is not used.  Just like with an audio power amplifier, the equivalent of a series RC Zobel network is often included from the output to ground.

The audio guys wanted larger parts like the 2SA1943 to get costs down but that is just good for us if we can repurpose them for high performance power supplies.  I tend to prefer smaller parts though because even if I have to use more of them in parallel, cooling is easier because the case to heat sink thermal resistance is distributed over a larger area and a lower total thermal resistance is possible.  On the other hand, this does make for more parts and layout complexity and an increased possibility of local oscillation if care is not used.

I have a list of the various On Semiconductor ring emitter parts intended for audio amplifiers and a quick check indicates that the D44H/D45H has about a 50% better ratio of power per dollar.  It would not surprise me however if they are also counterfeited more than the less common larger transistors.

I agree with T3sl4co1l though that these fast transistors are of marginal benefit in a general purpose bench power supply simply because it defeats the purpose of having excess high frequency performance at the end of a pair of test leads.  This does not apply for a point of load regulator and special purpose applications where the load is close.
T3sl4co1l:
Relevant example: in a recent design for a power supply, I wrote into the specification that the supply will have a maximum equivalent inductance characteristic.  The purpose of this is twofold: 1. the customer had been testing with external power, on leads of so-and-so length, therefore I can't do any worse than what is already known to work, if I meet this; and 2. the supply will have effective inductance due to control loop roll-off and output filtering, and this spec gives me a suitable (finite, nonzero) target to design to.

Tim
blackdog:
Hi guys,

I am aware that with a power supply without external sensing, I have little control over what happens after the wiring at the load.
Earlier I have made a comment about that.

And once again I want to point out that this power supply is for controlling precision circuits, as I indicated earlier.

Something like in the development of reference ovens, I want to have everything powerd from +15V.
So the oven heating and the reference, connected via short and twisted wires.
This happens in this way, both parts of the circuit go with separate wires to the power supply.
These parts are not electrically connected, except on the terminals of the power supply.

The heater can be a linear one, slow PWM(Arduino PWM) or a normal PWM circuit.
Normally i remove the fast edges on the pwm circuit, to get the noise down and it is easier on the power supply control loop.
And of course there is a lot of attention on decoupling.

A remark can be, that i can test with two power supplies, so i have no connection at all.
But that would not be the reality of the finished project.

Furthermore, it is not too difficult to bring out the sense wires with some extra security components to get better control at the load for short 4-wiring applications.

About the smaller power transistors, i did test this, but for 30V 5-Amps 4 of them where not enough to make it reliable.
I like the D45H D44H transistors, but nut at this power level, and again price is not a issue  ;D

Kind regards,
Bram
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