Lab power supply project - Pre-regulator re-design

The power supply design is now pretty close however I still have a few problems. I noted that at more than about 10V I can't get more than 4A of current before the output oscillates. I think the problem is that the maximum voltage the SCR pre-regulator's can achieve on the bulk capacitor is significantly less than the peak output of the transformer.

Also, no matter how hard I try to stomp on the ground issues I can't get rid of the last bit of noise on the output. The noise is a 1mV bump that occurs when the SCR fires to charge the bulk capacitor. The current flow causes the ground point to jump up a tiny bit and the output voltage to effectively drop by the same amount.

A while ago there was a discussion about low noise pre-regulators for linear supplies on the EEVBlog forum. A contributor called Blackdog described a pre-regulator that uses a P channel MOSFET as the pass element here on the EEVBlog forum. This design was picked up by another contributor Prasimix who is developing a 0-50V 3A supply here.

The Blackdog pre-regulator has a few advantages over what I am doing and may resolve some of my issues.

Blackdog Pre-regulator

The blackdog pre-regulator circuit is shown below:

The way it works is quite simple:

• Two P channel MOSFETs (Q2, Q4) are used to turn on or off current to the main bulk capacitor.
• The two AC inputs are combined and passed through an SCR (T1) and a resistor in series. The voltage on the SCR controls the voltage at the base of an NPN transistor (Q1) that drives the MOSFET gate. When the SCR fires the base of Q1 goes low.
• A PNP transistor (Q3) with a voltage divider on its base turns on when the bulk capacitor voltage reaches a certain proportion above the voltage regulator output.
• When Q3 turns on it fires the SCR which lowers the voltage at the base of Q1 and switches the MOSFETs (Q2, Q4) off.
• The capacitor between the base and collector for Q1 causes the gate voltage to ramp down instead of suddenly dropping. This helps by to reduce noise.
• When the voltage on the SCR falls (when we move to the next AC cycle). The SCR resets (commutates) which turns on the MOSFETs again and repeats the cycle.

So basically the circuit turns on the power to the capacitor at the start of each AC cycle and then turns it off when the capacitor voltage reaches the desired level. This has a lot of advantages over a traditional SCR pre-regulator

• The voltage/current ramps up with the AC waveform. The peak current flows when the bulk capacitor voltage is at a minimum and the transformer voltage is ramping up and charging the capacitor. The traditional SCR pre-regulator turns on when the transformer voltage is high and the bulk capacitor is low and creates a significant current spike. I think it is this spike that is creating the unwanted output noise in my current design.
• At the point where the MOSFET turns off, the capacitor has partially charged which means the current is lower than it would have been at SCR switch-on in the SCR design. The drop in current is changed to a ramp by the capacitor on Q1 which further reduces noise.
• It is much easier to figure out when to turn power to the bulk capacitor off than it is to figure out how long to delay turning it on. This makes the circuit much simpler.
• The Rds of the MOSFET is very low and results in a much lower voltage drop than the drop across the SCR.

Managing the Pre-regulator Voltage

The Blackdog circuit essentially sets the pre-regulator to a voltage that is some proportion above the regulator voltage. The thing is thought that the pre-regulator needs to be a constant voltage above the output regardless of the output voltage. The amount it is above the output is related to the maximal current demand and the bulk capacitor size as this will determine how much the voltage ramps down within a cycle.

In my version of this circuit I used one of the LT1716 comparators to compare the pre-regulator voltage with the output. I used a zener to set the pre-regulator voltage a few volts above the output.

It occurred to me that I needed this to be 5-6V above the output to ensure I could deliver 5A. If however the supply is not delivering this amount of current then the excess voltage just ends up as heat on the voltage regulator MOSFET. The bulk capacitor needs to he 1.06V above the output per 1A of current delivered. If instead we set the voltage to be some nominal amount above the output (say 1-2V) plus 1.5V per 1A (for safety) then I could massively reduce the dissipation.

The solution is to add a summing node to set the desired pre-regulator voltage and then use a comparator to fire the SCR.

The summing node adds 1.5 times the current to the required output voltage and will keep charging the bulk capacitor until the pre-regulator is another 1.4 (two diode drops) above that. Even going from 0 to 5A on the output this provides enough headroom to keep the output smooth.

The green trace above shows the current flowing (0-5A), the red is the output voltage (set to 14V) the blue is the pre-regulator voltage. You can see the pre-regulator voltage increase as the load increases and you can see the sawtooth waveform get taller as the current discharges the capacitor more per cycle.

One case where this didn't work as well was when the supply went into current limit mode.

In this case the voltage took one cycle to get back to the set point as the pre-regulator would discharge too low. I think this is acceptable however as current limiting is not a normal mode of operation but more a catch-all to prevent damage in case of operator error.

The power dissipation is relatively modest. With the output set to 14V and the load drawing 5A, the dissipation through each MOSFET is a series of short 12W spikes. This dissipation across the main pass transistor is around 22W (averaging about half that).

Conclusion

This pre-regulator looks like a winner. The next step is to build it up on my bread-board arrangement and test it out. 

Till next time!