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I would like to replace an Hbridge circuit that has 2 SPDT relays with MOSFETs.

The design parameters are as follows:

VBUS - 13VDC nom (12min 14max)
Icont - 50A
IPeak - 75A (intermittent still 3 mins so basically its still cont)
75A max On 5mins Off 3 mins

The circuit is controlled by manual operation that will operate a brushed DC motor FWD and REV. There is no uC and no PWM.

I have been looking at various MOSFET datasheets and have not been able to find any that would allow me to run VBUS into the Gate and have VDS just pass the current through.

I was looking at the Safe Forward Operating Area graphs and it seems like with almost any of the MOSFETs that a VDS of ~12VDC puts me extremely low on the available current allowed on avg ~10A.

For example http://www.vishay.com/docs/71903/sum85n03.pdf

On the safe operating area of the graph for a VDS of ~12VDC the ID would be ~8A if I am reading it correctly. Since there is no Micro I need to use DC values.

I can Change Vgs with a Voltage Divider but I cant change VDS. The only thing I can think of is to use parallel MOSFETs.

Any suggestions would be of help.

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  • \$\begingroup\$ The device you linked is a bit of a baby - have you looked for TO-220 packages with heatsinks? \$\endgroup\$
    – Andy aka
    Commented May 31, 2013 at 20:27
  • \$\begingroup\$ Take a look, for example at International Rectifier. \$\endgroup\$
    – Dave Tweed
    Commented May 31, 2013 at 20:41
  • \$\begingroup\$ Unless I am missing something even the datasheet for AUIRFS8409-7P shows the Safe Operating Area shows its still less than 10 Amps for a VDS of 12V? \$\endgroup\$
    – EE_PCB
    Commented May 31, 2013 at 20:54
  • \$\begingroup\$ Also this will be a medium volume application maybe ~1k a year. I am trying to replace 2 x CB1-P-12V, so I cant have the components that are chosen cost more than $7 for the 2 NFETs and 2PFETs. \$\endgroup\$
    – EE_PCB
    Commented May 31, 2013 at 20:57

2 Answers 2

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You are indeed misreading the SOA graph.

When you switch on a MOSFET in this type of application, the VDS is not equal to the supply voltage, it drops to a very low value, and most of the voltage appears across the load.

For this particular device, which has a worst-case RDS(on) of about 11 mΩ, VDS at 75 A is going to be about 0.85 V. Note that this falls right on the "limited by RDS(on)" line on the graph. You'll be fine as long as you can get the 62 watts of heat out of the package with a suitable heatsink.

Note that for a DC-controlled H-bridge like this, you will also need to select a similarly-rated P-channel device for the high-side switches.

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  • \$\begingroup\$ holy cow your right I see what I was doing wrong on the SOA graph. I get it now. I was also getting confused on where Rds(on) played a role. It is only calculated when the FET is in linear / ohmic / triode mode correct? once its in saturation then you should use the more complicated equation using transconductance to determine what Id can safely be. Should I limit VGS to 10V w/ a divider or a zener or just let it go (seeing as how its max is only 14) THANKS AGAIN! @Andy aka \$\endgroup\$
    – EE_PCB
    Commented Jun 1, 2013 at 12:36
  • \$\begingroup\$ In general, it's a good idea to limit Vgs to a value well under the absolute maximum rating of the device, especially in an environment where stray inductances (motors) and high currents can create some pretty nasty transients on your supply connections. \$\endgroup\$
    – Dave Tweed
    Commented Jun 1, 2013 at 12:41
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For this device, you won't get the heat from the device and you'll need a heatsink and TO-220+ package: -

enter image description here

If you drive the gate with at least 10V, the lower graph tells you the on-resistance is about 5 milli-ohms typically. At 50A this means a volt drop of 0.25V and to avoid going into the unsafe area of the graph (the part called limited by Rds(on)), your max current is about 20A.

Go for a bigger transistor not something that is surface mounted

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