Quad H-Bridge Design - Ideas

by Ivan Hamilton 9/4/2008 1:36:00 PM

I'm thinking about the quad H-bridge board I want to build to match my quad DC servo controller.

Standard thinking is a discrete bridge built from four MOSFETs and a pair of half-bridge drivers: 4 x IRFP260 @ ~$5 + 2 x IR2184 @ ~$3 = 200V 30A ~$26
But for lower voltages STMicroelectronics make the VNH3SP30-E a "Automotive fully integrated H-bridge motor driver": 1 x VNH3SP30-E = 36V 30A ~$8

These are used by the H-bridge boards I've currently got (packing a lot of power in to a small space). I was initially scared by SMD devices like this, but having seen people achieve amazing results without specialised equipment, I think it's worth the effort.

The only feature I feel is missing is current limiting. The VNH3SP30-E has internal thermal, over-voltage and over-current protection, but that won't stop it delivering 30A to your stalled 2A motor. Dropping a resistor between the VNH3SP30-E and GND would allow you to determine the current by measuring the voltage at VNH3SP30-E's GND pins and combining that with your knowledge of the resistor's value.

Let's use a low 0.01Ω resistor:

  • V = IR = 30A x 0.01Ω = 0.3V
  • P = I²R = (30A)² x 0.01Ω = 9W 

Therefore, for a 30A draw to show 0.3V, a 0.01Ω 9W current sensing resistor is needed: let's say 2 x 0.02Ω 5W resistors in parrallel.

For low current systems (<3A), the 0.03V drop may be too small to adjust. Let's use a slightly higher (but still very low) 0.1Ω resistor:

  • V = IR = 3A x 0.1Ω = 0.3V
  • P = I²R = (3A)² x 0.1Ω = 0.9W

A single 1W 0.1Ω current sensing resistor could be used in low current situations to make the drop more pronounced.

Add an LM339 to compare the 4 drops against a reference voltage and pass that thru a 4081 quad 2-input AND chip with the PWM signal for the H-bridge.

Piece of cake!

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CNC | Electronics

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Comments

11/16/2008 11:19:30 PM

Great work! But why don't you use the Rdson of the MOS on the pololu board to sense the current? It is connected as indicated on the application circuit on page 7 of the datasheet, so you can sense the current flowing to the ground on it's Rdson (~35mOhm if i remember correctly).

tinito

11/17/2008 9:35:18 AM

Nice point, but the 36V VNH3SP30 which I want to use, doesn't include the current sensing feature. The current sense feature is only included on the 12V VNH2SP30.

Ivan Hamilton

7/16/2009 3:01:26 AM

yes i actually read some process about molding..

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