# Dynamic Braking resistor chopper circuit

I want to design a standalone dynamic braking circuit for AC motor driver , because to deal with regenerative energy that the AC source cannot handle causing a DC bus voltage Rise . The concept is well established in many resources online that senses the DC bus voltage and dissipates the access energy in a " Brake Resistor ".

I want to know what is usually used circuit that is called in the diagram as "chopper module" and design it without micro controller if possible.

I donot know of an IC that is designed for this purpose solely , so what comes to mind is comparator with hysteresis with a low side gate driver to open/close Braking mosfet. but no so sure if this is a decent choice until i test in reality and see how well it will work.

My concept test circuit :

So I would like to know your comments on the subject, and corrections if i am missing something here .

• I imagine that you would want to increase PWM from 0 to 100% as the DC bus voltage rises from $V_{THRESHOLD}$ to $V_{MAX}$. Your comparitor gives all or nothing. – Transistor Jul 19 '17 at 17:27
• It would work like a relaxation oscillator, FET on pulling the DC bus down, then off and the braking pumps it up again. As long as the resistor is rated to dissipate the full braking power, this looks just fine. You might want a FET gate driver that turns on and off faster than that 1k pullup, to reduce your FET dissipation during the transitions. – Neil_UK Jul 19 '17 at 17:35
• @Neil_UK , for sure , i will be using a gate driver to drive the mosfet something like MCP1407 , the 1k will be driving the input of MCP1407 . – ElectronS Jul 19 '17 at 17:39
• @Transistor , so what do you propose ? have you designed such circiut ? – ElectronS Jul 19 '17 at 17:39
• Nope. I'm an industrial user. Your simple circuit might work well enough. – Transistor Jul 19 '17 at 17:42

What you are presenting is an extremely common method to deal with the regenerative energy. It is a simple chopper circuit to regulate the DClink voltage via dissipating the energy in the resistor

Whether the hysteresis is done in the hardware domain (your comparator) or in software comes down to system architecture. I have personally done both

1. Please place a diode across the brake resistor.

The inductance of such a resistor & leads can produce a destructive voltage. There is always stray inductance, but aspects of power electronics can cause the "inductive kick" to be a real problem. Lead lengths tend to be longer & equally the current involved higher. Both these contribute to a higher V due to $L\frac{\Delta I}{\Delta t}$

2. Consider a second comparator for an over-voltage.

If the brake circuit fails you could cascade a failure during deceleration. Overvolting power-electronics has a habit of exploding. Imagine the situation where your brake circuit failed (undersized resistor, random failure of the resistor, FET, comparator etc...). Your controller will still decelerate transfering the inductive and rotational energy to the DClink. Normally this would be regulated by the chopper, however this no longer exists & the DClink will carry on rising as long as there is energy to be transferred. At some point something will reach its avalanche voltage: Inverter or Capacitor. The energy stored in the capacitor, $\frac{1}{2}CV^2$ will now rapidly dissipate & further failures can & do occur.

• as for comment 1 , i have seen this diode , thank u for clearifying why it is used , i will added it for sure. as for consideration 2. , i didnot understand it , cascad what and how ? – ElectronS Jul 19 '17 at 17:41
• I have added a bit more information to try to explain the concerns about fault propagation, especially under this situation – JonRB Jul 19 '17 at 17:56
• thanks its more clear now , the second comparator will present redundancy , but this time to shut down the controller , no to connect a brake resistor if i under stand you correctly. – ElectronS Jul 19 '17 at 22:03
• exactly. What you have to remember is the higher voltage is due to you decelerating the rotor. If you disable (momentarily) then the source of the overvoltage will be suppressed. It system dependant & with such functionality being done in Hardware the associated BOM/cost needs to be considered. In software/firmware its an easier decision. – JonRB Jul 19 '17 at 22:26

I want to know what is usually used circuit that is called in the diagram as "chopper module" and design it without micro controller if possible.

Basically it is a shunt regulator - if the DC bus voltage rises to a critical point the load resistor is placed across the bus until all the dumped energy is turned to heat but there can be more sophisticated versions and this one, from its description, sounds like one of those.

I suspect that as the voltage rises above a certain level the MOSFET is progressively turned on and off starting at a low duty cycle and if the bus is still rising the duty cycle gets bigger and ultimately it will be switched on 100% until the energy is dissipated. The MOSFET is therefore driven efficiently and all the heat is dissipated in the resistor.

So, what you are looking for is a PWM circuit than can drive the MOSFET and a sense circuit that "examines" the bus voltage in order to determine what the duty cycle should be.

• Examination is best done by OP amps , i think ? .. PWM cirucit , maybe an 555 timer ? .. i wish there was an app note on the subject – ElectronS Jul 19 '17 at 17:44
• Yes, an op-amp can be used that say provides an output voltage from zero volts up to say 1 volt where 0 volts means zero duty and 1 volt means 100% duty. This can be used to control an LTC6992 (i.stack.imgur.com/Jnhsc.jpg) and ultimately this can switch the MOSFET on and off via a suitable driver. You could use a 555 but it's a bit more fiddly. – Andy aka Jul 19 '17 at 17:46
• good suggestion , i didnot know about LTC6992 before . thanks – ElectronS Jul 19 '17 at 17:51