DHT LT power supply PCB tests

I’ve been working on a few PCBs lately with great results. I started with an HT power supply PCB which allows the use of full silicon rectifier bridge, hybrid rectifier (e.g. for damper valves) and any combination of either CRCRC or CLCLC stages with external chokes. Last capacitor is bulky WIMA DC-Link film instead of electrolytic. That worked really well and I’m using it now in a +50/-300V supply for the fixed bias and source follower drivers in the amps.

Then I moved to modify the design for a low-tension (LT) supply. This is much needed for any DHT stage and could also be used for any sort of filament supply.

With the same concept in mind, I wanted to have the flexibility to use (or not) external chokes. You need to get enough space for the “snap” type of electrolytic capacitors (like the great quality KEMET 22mF / 25V). Here is the generic PSU circuit topology:

You can see a classic snubber formed by C1 and R2. I use low-drop Schottky diodes (in TO-220) albeit there is an option to use classic diodes as well. First cap (C2) is the tuning cap (choke-input vs. cap-input) and then you can either use L2 and L3 or resistors R1 and R4. R1 and R3 provide the input resistance to main bulky cap C3. Then you have C4 before the common mode choke L1 (I will rename it as L3 to make it consistent). C5 is the second bulky electrolytic cap. Then you have room for a 100nF and a 220pF ceramic cap for HF. There is a bleeder resistor (R5), the on-board fuse (F1) and the indicator circuit, an LED plus its resistor.

Here is the board before assembling:

And here is the board finished:

Ok, here is the circuit I tried. I wanted to avoid the external chokes on this first test, so here it goes:

I have a nice set of 100VA JMS transformers with split bobbins and shielded with copper screen. It does have taps at: 0-0.5-1-2.5-5-7.5-10-20V and all these options allow me to configure the transformer to the desire output voltage.

I made a few tests with 0.9A and 1.8A loads and output ripple was between 0.7mV and 1mV. Really good!

I may run these PCB in a production batch. However, I don’t have time but more importantly I don’t have the time to answer emails of people asking me for help in designing their power supplies. I don’t know, perhaps I’ll do it.