Problems with freewheeling diodes in MOS drive motors

我本将心向明月

Problems with freewheeling diodes in MOS drive motors [Copy link]

 

As the title says, and as shown in the figure below, when using MOS as a switch to drive a DC motor, a freewheeling diode is added to avoid damage to components caused by induced electromotive force. What I am more curious about is, as shown in the figure below, whether the two diodes D10 and D11 have the same function. I personally think that D10 can meet the requirements, so why add D11? Can anyone explain?

zxopenlyt

Benefit

maychang

"As shown in the figure below, do the two diodes D10 and D11 have the same function? I think D10 can meet the requirements, so why add D11?"

Can't see the picture.

我本将心向明月

maychang posted on 2021-3-16 18:16 "As shown in the figure below, do the two diodes D10 and D11 have the same function? I personally think that D10 can meet the requirements. Why add D11...

I don't know why the picture was not deleted, I uploaded it again

maychang

I originally directed my heart to the bright moon and published it on 2021-3-16 20:45 I don’t know why the picture was not deleted, I uploaded it again

"Personally, I think D10 can meet the requirements, why add D11?"

I also think that D10 can meet the continuous current requirement. D11 is redundant.

maychang

I originally directed my heart to the bright moon and published it on 2021-3-16 20:45 I don’t know why the picture was not deleted, I uploaded it again

Moreover, there is a parasitic diode inside the power MOS tube (not marked in the figure, but it will definitely be marked in the tube's instruction manual).

This parasitic diode is connected in parallel with D11 in the figure (in the same direction). Therefore, if D11 is removed, the parasitic diode inside the power MOS tube can still play the role of D11.

我本将心向明月

maychang published on 2021-3-16 20:51 Moreover, there is a parasitic diode inside the power MOS tube (not marked in the figure, but it will be marked in the tube manual). This parasitic diode, ...

Your reply gave me some ideas, thank you.

I think D10 and D11 provide protection from two perspectives.

1.D10 plays a protective role from the perspective of continuous current.

2. D11 is to prevent the drain voltage from being too high, which may cause the MOS tube to burn out, so a TVS diode should be used. As for why the parasitic diode inside the MOS tube is not used, it is probably because the parasitic diode is produced by the production process, so the performance is not as good as that of a dedicated TVS diode. At the same time, if the voltage is too high and the internal parasitic diode is permanently damaged, the MOS will still be damaged, and it will not play a protective role.

PS: There are reports online that the D11 position also has a freewheeling function. In my humble opinion, this position does not have a freewheeling function, or it may be able to freewheel under certain conditions, which I have not thought of. I hope you can give me some reference opinions. Thank you.

仙猫

The speculation is that the MOSFET symbol on the original drawing originally had a body diode in it, but the person who copied the drawing didn't know it was the same device and wanted to copy it, but ended up drawing two different devices.

仙猫

maychang

I originally will look to the moon at 2021-3-16 21:34 maychang at 2021-3-16 20:51 Moreover, there is a parasitic diode inside the power MOS tube (not marked in the figure, but the manual of the tube states that...

『1.D10 plays a protective role from the perspective of continuous current.』

This is correct. When the MOS tube is turned off (the direction of the self-inductance electromotive force of the inductive load is negative at the top and positive at the bottom), D10 is inductor freewheeling.

maychang

Xianmao published on 2021-3-17 09:10

"The person who copied the picture didn't know it was the same device, so he tried to copy it, but ended up drawing two gourds."

It is entirely possible.

maychang

I originally will be with the moon at 2021-3-16 21:34 maychang posted at 2021-3-16 20:51 Moreover, there is a parasitic diode inside the power MOS tube (not marked in the figure, but the tube manual says...

"2. D11 is to prevent the drain voltage from being too high, which may cause the MOS tube to burn out, so a TVS diode should be used."

The induced electromotive force can only damage the MOS tube when it switches from saturated conduction to complete shutdown. During the conduction period, it is impossible for the MOS tube to be damaged by excessive drain voltage, but it can only be damaged by excessive current.

When the MOS tube switches from saturated conduction to full shutdown, as you mentioned in 1., the direction of the electromotive force at both ends of the load is negative at the top and positive at the bottom. If there is no D10, this electromotive force can be very high. Moreover, the direction of this electromotive force is superimposed in the same direction as the power supply direction, which may be much higher than the power supply voltage, thus breaking down and damaging the MOS tube. However, due to the existence of D10, the drain voltage of the MOS tube is only the power supply voltage plus the forward voltage drop of D10 (ignoring the parasitic inductance of the connecting wire). Therefore, with D10, the drain voltage of the MOS tube will not be too high when the MOS tube switches from saturated conduction to full shutdown.

So, D11 is totally useless.

maychang

I originally will look to the moon at 2021-3-16 21:34 maychang at 2021-3-16 20:51 Moreover, there is a parasitic diode inside the power MOS tube (not marked in the figure, but the manual of the tube states that...

The previous reply ignored the parasitic inductance of the connecting wire from D10 to the motor load. If the parasitic inductance of the connecting wire is relatively large (the inductive load and D10 are far away from the MOS tube), then the MOS tube may indeed be broken down when it is completely turned off. In this case, it is beneficial to connect a TVS tube in parallel with the MOS tube. From this we can also know that D10 should be placed near the MOS tube, and not at both ends of the inductive load.

You said: "In my humble opinion, this position does not play a role in the flow of electricity", which is correct.

我本将心向明月

Xianmao posted on 2021-3-17 08:59 The speculation is: the MOSFET symbol on the original picture originally had a body diode, and the person who copied the picture didn't know that it was the same device, so he wanted to copy it, but ended up drawing two different things.

This is a good idea, and it is possible

If I replace my D11 with a TVS diode, I think it can also play a protective role, such as anti-static. But is such a design necessary?

我本将心向明月

maychang published on 2021-3-17 09:56 "2.D11 is to prevent the drain voltage from being too high, which may cause the MOS tube to burn out, so a TVS diode should be used. " The induced electromotive force has an impact on the MOS tube...

Let me summarize, so my above summary is correct

1.D10 should use a freewheeling diode

2.D11 should use TVS diode

Therefore, when selecting these two tubes, we must distinguish them based on their usage. D10 is necessary, while D11 is the icing on the cake (because the parasitic diode inside the MOS can play a certain role).

In terms of the explanation of the principle, maychang above gave a very detailed explanation. I would like to express my gratitude here. If there are any errors in my explanation above, please point them out and I will make corrections.

maychang

I originally will be with the heart towards the bright moon and published it on 2021-3-17 10:05 If D11 is a TVS tube, it can also play an anti-static role from the product characteristics. According to your experience, is it necessary?

The gate of the MOS tube needs to be protected from static electricity, because the gate is a very thin layer of silicon dioxide to the source (including the drain, or more precisely, the channel), and its withstand voltage is only 20V (20V is specified in the manual, and the actual measurement does not exceed 30V), which is very easy to be broken down. As for the drain, at the rated voltage, the leakage current to the source can reach several microamperes or more (see the MOS tube manual). Such a large current can completely release the static electricity generated under normal circumstances.

Therefore, unless the environment is extremely bad, I don't think it is necessary to connect a TVS in parallel with the MOS tube. If the environment is extremely bad, it is useless to connect a TVS in parallel at both ends of the MOS tube. In that environment, protection must be done from the shell to the external interface.

我本将心向明月

maychang published on 2021-3-17 10:16 The gate of the MOS tube needs to be protected from static electricity, because the gate is a very thin layer of silicon dioxide to the source (including the drain, or more precisely, the channel), and its withstand voltage is only...

Been taught a lesson

shaorc

I originally turned my heart to the bright moon and published it on 2021-3-17 10:22 I have learned a lesson

Hard core popular science, pure dry goods, thank you for sharing

se7ens

D11 is indeed superfluous, the motor's freewheeling only requires D10

The reply on the 6th floor seems to make sense.

田国瑞

Learned