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I have used this circuit (I found online last year) to drive big dc motors without much thinking but with great success. http://obrazki.elektroda.pl/4961105700_1437649469.jpg : -

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But yesterday I noticed something; there are 3 TVS (two from each motor terminal to GND and one from VCC to GND) BUT THEY ARE BIDIRECTIONAL

Now i am thinking, isn't it better to use unidirectional instead in such circuit?

Another aspect, should I add 2 more TVSs across the high side MOSFETs also?

Please give your opinion, maybe these diodes are not needed at all?

NOTE : i am using locked anti-phase or sign magnitude drive methods WITH synchronous rectification.

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Another aspect, should I add 2 more TVSs across the high side MOSFETs also?

It could be argued that the only TVS needed is the one between 24V and ground. The diodes inside the MOSFETs will steer any back-emf from the motor to the rails - think of these four diodes as a bridge rectifier (because that is exactly what they are) and think of the motor as the secondary of a transformer. The output of the bridge clamps the voltage thus protecting the MOSFETs (which happen to be across the four diodes).

The back-emf from the motor will be limited to the current in the motor and so providing the diodes are rated sufficiently above at the maximum current that can be taken by the motor there should be no tears.

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  • \$\begingroup\$ So instead of TVS across the 4 mosfets , 4 schotkey diodes with their "pulse" NOT contiuons rating same as the bridge 50A for example , should be suffcient . I will keep the tvs across the 24v and GND , but you didnot specify if it should be UNI or BI directional \$\endgroup\$ – ElectronS Jul 23 '15 at 16:18
  • \$\begingroup\$ The power supply TVS can be unidirectional BUT you don't normally need to add diodes external the MOSFETs because the MOSFET process inevitably produces a parasitic diode that does the job. Read the data sheet on the device and see what I mean. Hopefully it will be rated suitably. \$\endgroup\$ – Andy aka Jul 23 '15 at 16:37
  • \$\begingroup\$ i know about the intrinsic body diode , but i thought i needed external ones , when you said the diodes should be rated sufficiently . I also mentioned about the driving mode of the H bridge being synchronous , so hopefully the bodydiode will not conduct too much , since the mosfet will do the freewheeling , thanks for your help ( i donot know new users like me couldnot rate your answer as useful , so other could benefit also) \$\endgroup\$ – ElectronS Jul 23 '15 at 17:41
  • \$\begingroup\$ Don't worry about it - you need to get a rep of 10 before being allowed to upvote and you're nearly there. \$\endgroup\$ – Andy aka Jul 23 '15 at 21:45

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