Design flaw in pololu shunt regulator? - Page 1
Forum section: electronics / projects

author: andyw
author_rank: Newbie
timestamp: December 18, 2020, 07:54:51 am
content: Dear all,I've bought three "Pololu Shunt Regulators": 26.4V, 2.80Ω, 15Whttps://www.pololu.com/product/3775/Two of them already got damaged while freewheeling of a 40W stepper motor with approx. 100J of kinetic energy. 230V TDK power supply 3-> 24V/3.2A -> shunt regulator parallel to motor driver, shutting down 230V while motor is rotating. In this case the voltage on the driver shoots >30V due to regenerative mode of the motor.In both cases it looks like the MOSFET got damaged. I've tried to make a sketch of the schematic (see attachment) and made some measurements on the remaining "working" device:If the VIN Voltage rises above 26.4V, the Gate-Source Voltage of the MOSFET (AON6232) is only 2,4V which results in VDS approx. 24V so the MOSFET has to dissipate 90% of the power (and thus getting hot really quickly) and the shunt resistors only 10%. I really doubt this was intended by the designer.So the 15W which are stated in the product description actually heats the MOSFET and only 2W are dissipated on the shunt resistors.Btw, can someone tell me what device is the one labeled wit “GC9” I guess a voltage detector with 2.5V threshold (something like ROHM BD48L25) but I can’t find a device with SOT-23 package which would match the used connection.Thank you very much for reading (and perhaps helping)--Andy

author: Niklas
author_rank: Frequent Contributor
timestamp: December 18, 2020, 09:03:02 am
content: The GC9 part looks like a TL431 shunt regulator. If the voltage on the adj-pin goes above 2.5 V it will start to sink current.

author: Syntax Error
author_rank: Frequent Contributor
timestamp: December 18, 2020, 10:08:04 am
content: Please refer to following response

author: Ian.M
author_rank: Super Contributor
timestamp: December 18, 2020, 10:36:51 am
content: Try adding a little hysteresis - a 10 Meg resistor from the MOSFET gate to the feedback pin of the GC9 shunt regulator should give you about 100mV#between the turn-on (upper) and turn-off (lower) thresholds so once it triggers at the upper threshold the MOSFET turns hard on till the voltage drops below the lower threshold then turns fully off, repeating as necessary if current continues to be dumped into the rail rather than simmering in its linear region as it does currently.The remaining limitation will then be the on-board 'shunt' (brake) resistors - if you anticipate need for frequent braking, you may need to disconnect the on-board brake resistors and fit a suitably heatsinked external one.# I'm guessing the gate clamping Zener is around 15V and assuming the divider resistors on your schematic are correct and that you are using the 26.4V version that does *NOT* have an on-board adjust pot. If you are using the Adjustable HV version, you'll get a bit less hysteresis.@Syntax Error: Polulu says the pot on the adjustable versions is 200K.

author: andyw
author_rank: Newbie
timestamp: December 18, 2020, 11:43:58 am
content: @Syntax Error: I'm sorry, I've written "optional" above the pot to indicate that there a holes for a potentiometer but it isn't placed. My bad to not make this clearer. So as Ian.M already mentioned: The 26.4V variant hasn't a potetniometer, only the voltage divider with 64,9kOhm and 6,81kOhm (which results in 2,5V VRef when VIn=26.4V)

author: andyw
author_rank: Newbie
timestamp: December 20, 2020, 05:42:28 pm
content: Thanks to Niklas which the hint to TL431 I made a LTSpice simulation (see attachments) which pretty much matches my measurements on the remaining device.It looks like the MOSFET is operated in the linear mode and has to dissipated much too much power.Ian.Ms idea to give some positive feedback to give some hysteresis clearly would help.Thank you all.