Universal DRSSTC Driver 2.7

Here is my take on Steve Ward’s Universal DRSSTC Driver 2 which I have named UD2.7. The UD2 is THE benchmark DRSSTC driver: it has stood the test of time and been used in thousands of high performance coils around the world. I came across Steve’s original UD2 a while back ago, but there were several new features I wanted to incorporate. In addition, I thought it would also be a good idea to share a little bit about how the UD2 works and how to get one working, with the Tesla Coil community. Hopefully this will be a useful resource to hobbyists around the world. The UD2.7 (now in revision C) has since been used successfully in a variety of coils, ranging from small table top 400kHz DRSSTCs, 35kHz large DRSSTCs with a full bridge of CM600 IGBTs and even QCW operation (see application notes below).

This project will not be possible without the work of the following people. Many thanks to Steve Ward for his generosity in sharing the UD2 with the Tesla Coil community, and for his permission for the modification and publication of this new version of the Universal Driver. The UD2.7 is free for download for personal hobby use; credit and all rights of the UD2 design goes to Steve Ward. Also, credit goes to the many improvements made by Bart and Eric in the earlier V2.1 and V2.5 revision, which added lots of cool features.

At the moment I’m quite happy with the UD2.7C revision, and any new features will probably be incorporated in a completely new driver design I will be working on in the future 😃. Feel free to drop me a note if you’ve successfully used this in your project or if you have any neat suggestions or comments!

See this 4hv forum thread for questions, discussions and answers. This page will also be continually updated.
If there are additional changes desired / comments / feedback, please let me know for a future revision of the board.

Quick Note

24VDC Input - Small error in the UD2.7A and B design which has been fixed in the UD2.7C; also does not apply if you are only using the AC input jack. Otherwise, there is a simple fix if you are using the 24VDC input jack for the Rev A and B boards: solder on a general purpose diode across the 24V regulator from output to input (i.e. standard reverse protection diode). See below for more details.

C33 - C33 sets the dead time for the TL3116 comparator hysteric feedback network for switching noise immunity. The pads for the capacitor are increased in size for easy changing of components. Good values range from 220pF to 2.2nF. For large IGBTs at low switching frequency (e.g. CM600), use 2.2nF. For medium sized switches like CM200, 1nF works well (change C33 to less than 1nF for operation above 200kHz). For faster small IGBTs such as TO247 IGBTs at higher frequencies, use 220pF. Adjust C33 if noise is found on the output of TL3116.

Fiber Units - The UD2.7 allows the use of two different kinds of fiber optic input (the ST-type, and Industrial Fiber Optics type). Use OPF-2412T or HFBR-2412T for ST fiber receivers, & Industrial Fiber optics IFD95T or IFD95C (these are Active-High, be careful that you do NOT buy the active low kinds). The fiber units are ORed together, so be sure to ONLY use one port for interrupter input to avoid excessive pulse-widths. They can be installed individually by themselves to save cost.

D13 - You can install both, only the ST, or only the IF receiver to save cost. However, because the IF receivers are active high, if you only install the ST receiver, you must remove D13 to prevent the interrupter line from not being pulled low.

Slot-7 Inductors - These 7M3-series tunable inductors for phase lead adjustment can be bought or sampled from www.coilcraft.com. Inductance values varies depending on your switch and switching frequency. Typical values include: 7M3-123 (9 - 15uH; works well with TO247 IGBTs), 7M3-153 (11 to 19uH), going up through 7M3-223 (17-28uH), 333 (25-41uH), 393 (29-49uH; works well with CM200/300s), 563 (42 - 70uH) etc. Use a flat-head non-magnetic or plastic screwdriver for tuning.

Output Drive - UD2.7 has two identical outputs for driving two bridges, or can be paralleled to run a large bridge. To save cost, the second gate drive section can be removed - anything after the phase jumper can be removed for the second circuit, including the UCC Mosfet drivers and the push-pull MOSFTs.

Feedback - I try to aim for about 1A of feedback current; adjust your CT turns ratio appropriately.

OCD Burden Resistor - The OCD burden resistor (R17) is a single 5.1R 2W by default. For flexibility, an second parallel space at R28 is provided for an optional secondary resistor. The comparator voltage should ideally be less than 7V to keep within the bounds of the comparator when on a 9V rail.

Setting OCD Current Limit / Phase lead - See application notes below.

UVLO Setting (for UD2.7B onwards) - UVLO trips when the voltage sense line drops below 2.7V (or 4.5V depending on the part you use). This is set in UD2.7A with a 33k/4.7k resistor divider across the 24V rail giving about 3V, for the UVLO to trip when the 24V line drops to 21.65V or so. In UD2.7B onwards, R29 is a 50k pot which can be adjusted greater than 33k for a faster trip, or a less than 33k for more leeway in 24V drop. Tap point 2 (TP2) is conveniently located near R29 for easy voltage adjustment. This can be set to 3.0V (or 5.0V for the 4.5V trip). Note that the maximum voltage the LM8365 can tolerate is 6.5V. By default, R29 (50k pot) comes set to half (25kR resistance). This gives a safe voltage of around 3.8V by default which should be adjusted. However, IF R29 is set to say 0R, this can place a very high voltage (>6.5V) to the LM8365 causing it to pop! Be careful when installing R29!

UD2.7 List of Improvements

• Heat-smearing 9V regulator replaced with an efficient switching regulator (but still TO-220 compatible if you want, with space for heatsink!)
• All other regulators replaced with SMD ones, reducing overall height of the board
• Added on-board SMD LED for power indication
• Under-voltage Lockout added!
• Flattened ground plane and some re-worked traces and component placing
• Added new 24VDC input jack for use with small DC power supply instead of power transformer
• Improved solder pads for hard-to-solder components such as the dual MOSFETs and some others
• Added series capacitor with 15R resistor in parallel with 2uF GDT output capacitor to prevent burni ng out resistor in event of malfunction
• Improved silkscreen for easier use (e.g. LED breakout jack now labeled on top)
• All through-hole resistorsare now flat instead of standing up, for increased robustness
• Some components changed to further reduce height of populated board
• Board edges rounded, mounting holes changed to circles, board size shrunk to be even more compact
• Main component reference IDs (e.g. C33) maintained for easy discussion on forums
• Includes all improvemen

QCW Operation

ts from UD2.5 by Eric, such as added IF fiber optic input, test point for OCD, and fixed errors

• Created BOM, documentation, and improved schematic
• New compact size of 2.4 x 3.7" footprint

More improvements for Rev B & Rev C

• Improved silkscreen and component placing

• Additional optional gate-drive capacitor for use in high pulse-width applications (optional)

• Added UVLO adjustment potentiometer for lock-out adjustment (or you can use a resistor instead for a fixed one) and UVLO jumper

• Improved heat-sinking capabilities of 24V and 5V linear re gulators with less thermal relief

• Fixed error on 24VDC rail with now higher AC input range of 19-26.5VAC nominal and 28VAC max

• Yet more improvements to silkscreen including diode direction labels and other cosmetic improvements

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