Snubber for solving a heat problem in a DC motor control

[seyyah]

I have a mosfet that controls a dc motor. Without snubber, mosfet gets hotter and hotter. With a diode snubber, this time diode gets hot. I used a slow diode. I'll also try a fast diode but i wonder that does snubber helps to reduce the dissipated power or does it take it from mosfet onto itself? I mean, must i use a huge diode or can i reduce component(snubber) size?
Do you have another suggestions that solves this heat problem?

 

[batdin]

diode snubbers

My personal experience with RC snubbers shows that they DECREASE efficiency.The positive effect is that of suppressing the voltage spikes thus protecting the transistors or diodes.Lossless diode snubbers should not suffer this efficiency problem.BUT only if the diode is fast.

 

[mostafa0020]

lossless snubbers

I think MOSFET is the least power electronic switch that requires a snubber circuit!!

 

[nicleo]

snubber+diod+size

Snubber circuits are used to modify the switching waveforms of power semiconductor switches. In general, snubbers can be divided into 3 categories:

1) Turn-on snubbers to minimize large overcurrents through the device at turn-on.

2) Turn-off snubbers to minimize large overvoltages across the device during turn-off.

3) Stress reduction snubbers that shape the device switching waveforms such that the voltage and current associated with a device are not high simultaneously.

Usually a power mosfet should be operated correctly in its Safe Operating Area (SOA). A power switch rated as 60A does NOT mean that it can be operated safely at 60A in any condition. Figure below is the SOA curves for NTP60N06 (rated at 60V and 60A). Allowable currents, as shown in the SOA plot, are time-dependent, that is, for shorter durations higher currents may be conducted. The transition between ON and OFF and across the ID - VDS region should be fast to avoid large switching losses. The transitions into high power regions of the upper right hand side of the SOA are time limited. On the other hand, the mosfet can be operated in the lower left hand region continuously. Besides operating properly according to SOA, the driving circuit of the power mosfet should be designed properly to make sure that the power semiconductor works as 'SWITCH' (can check with scope). Proper heatsink design is also important to make sure that the power mosfet works within its thermal limit.

 

[ee_wmkab]

mosfet snubber

Dear seyyah,

The snubber diode should be in forward bias during turn on/off of your mosfet. It is used to prevent transient (overstress) voltage to the mosfet which may damage the mosfet.

If you found your mosfet (i guess it is used to turn on/off your DC motor) hotter and hotter during the DC motor on, It may due to the high current is drain during in this peroid. If the mosfet is very hot, it may over the SOA(safty operating area) of the mosfet. DO check the spec of the mosfet and add heat sink on it (or change to high power mosfet).

BR
Keith

 

[bunalmis]

snubber circuit for power mosfets during turn on

Do you true calculate the snubber companent's values for selected frequency?

 

[fsmyth]

rc snubbers and diode snubber

Seems that everyone has neglected to mention that
power MOSFET's have an inherent body diode; it
could very well be that the heat is originating from it.
Quite a lot depends on the circuit, the FET drive, and
other factors, but as a general rule, a faster (snubber)
device should be added for spike protection when the
back EMF approaches the FET ratings, or to absorb the
reflex and allow more control over the FET switching.
<als>

 

[cesare]

dc motor snuber

Snubbers can be used for several purpose:

1) they can be used to reduce stress on the switch element during the switching by diverting the current from it.

2) they can be used to reduce the Q of the resonant circuit resulting from the parasitics in the circuit (transformer leakage inductance and mosfet drain capacitance). This can help in meeting EMC / EMI requirements.

In every case the snubber dissipates energy and it reduces overall efficency.

For a good AN on the topic see SLUP100 on TI seminar SEM900 available on TI site