Universal DRSSTC Driver V2.5

Rework by Eric Goodchild (Goodchild Engineering)

Have a question? Check the 4hv forum post on the UD2.5 

Board files/eagle files >>> UD2.5  

V2.5 REV A

In the first Rev of the UD2.5 the pull-down resistor on the diode OR junction that merges the two fiber inputs together is not included. To fix older boards, simple kludge a 1k pull down resistor from the AND/diode output to ground as shown in the updated schematic (R21 for reference


The UD2.5 is an updated/reworked version of Steve Ward’s popular Universal DRSSTC driver with phase lead. I added many changes and fixes.

The UD2.5 is pin, mounting hole, and electrically compatible with the older UD2.0 and UD1.3 style drivers. It can be dropped into an existing system that uses the UD2.0 or UD1.3 with no modification.

PCB/Board Gerber files:

Top Copper (GTL)
Top Soldermask (GTS)
Top Silkscreen (GTO)
Bottom Copper (GBL)
Bottom Soldermask (GBS)
Bottom Silkscreen (GBO)
Drill File (2:4 leading) (TXT)
Top Paste (GTP) (Optional: used for solder paste stencil for reflow assembly)

Fiber RX Units:
ST style HFBR-2412T or OPF-2412T & Industrial Fiber optics IFD95T or IFD95C are both supported.
Both fiber optic units can be installed at the same time and run in tandem. (i.e. they will have OR operation) Warning, do not drive both fiber RX units at the same time which can result in extra long PW. If desired, either one of the fiber RX units can be left off to save parts cost. However, both pull-up resistors (R10 & R11) should always be installed for noise immunity.

NOTICE: If you plan to not install the industrial fiber optics unit (IFD95T or IFD95C), keep R11 installed and remove D13.  R11 is a pull-up resistor that allows the use of the open collector type units (IFD95C) and the push-pull units (IFD95T), but because they are active high, if you don't have it installed it won't pull the interrupter line low. As a result D13 must be removed so that the interrupter line won't be on constantly. 

Phase Lead:
The phase lead selection jumper (SV1) at the board front-end allows users to select between having the phase lead inductor in the circuit or using just a resistive burden in the feedback (this is useful for debugging).

The inductor needed on the board will vary with the fres of the coil and the speed of the power switches used. Removable pin headers have been added for easy inductor swapping. After the correct inductor is found it may be soldered in if desired.

Commonly used inductors can be found at

The SLOT TEN (10mm through hole inductors) series of variable inductors was typically used in the UD2, but I've now changed over to the 7mm inductors. I mainly did this because they're easier to work with when tuning them. They can be adjusted with a simple non-conductive, small, flat head screwdriver (an O-scope probe adjustment screwdriver works great!).  This was a happy accident; the first run on the boards had a 7mm inductor footprint by mistake. However after testing with 7mm inductors I found that I actually liked them better.  

Typical Values I have used:

Part #                        min(uH)      nom(uH)      max(uH)






The capacitor (C33) in the hysteretic feedback network of IC14 (TL3116) may also need to be adjusted
depending on the speed of the power switch used. C33 (set at 1n) sets a high noise immunity dead-band
during transition of the power switch for higher noise immunity in the feedback network. 1nF is a good
general value for medium to large sized switches such as CM200, CM300, SKM400, SKM300, etc… Larger
switches such as CM600s may need a larger value. Some people report a needed value of up to 2.2nF for
CM600 style bricks. If noise is found at switching transitions at the output of the TL3116, consider
changing the value of C33.

GDT driver section:
To save soldering time and cost, the bottom gate drive section (GDT2) can be left off completely. All
parts in the gate drive can be removed including the UCC gate drive chips (all parts after the phase

General changes/fixes from UD2 and UD2.1:
  • Fixed PCB error in TL3116 feedback network
  • Linked schematic and PCB files in eagle
  • Added support for industrial fiber optics IFD95T or IFD95C fiber RX units
  • Changed GDT primary damping resistor to larger through hole for better heat dissipation
  • Chained 5Vreg input to 9Vreg output for better thermal performance in 5Vreg
  • Rotated all Vregs 90 degrees for easy heatsinking
  • Added socket system for phase lead inductor for quick swap
  • Changed phase lead select header to allow for LR or just R burden network (for debugging)
  • Added optional second parallel burden resistor in OCD network for larger coils
  • Electrolytic capacitors changed to through hole parts
  • LED header labels on underside of PCB
  • PW limiting circuit found in UD2 has been removed
  • Test points added for easy reading of OCD set point voltage (TP1)
  • Inductor changed to 7mm

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