Max current spikes in diodes

In summary, when charging a capacitive load with a low impedance source through a diode, there is a high current spike through the diode while the capacitor charges. Most diodes have small peak pulse currents, but can handle surge currents due to their specifications. The in-rush current to the filter capacitor in power supplies is extremely high, but rectifier diodes have a higher surge rating than average current rating. The key is selecting the diode that best fits the needs of the circuit. In cases of large capacitances, additional measures may be necessary to keep the current at a tolerable level.
  • #1
830
1
If I consider a simple example of charging a capacitive load with a low impedance source, and charge the capacitor through a diode, I will have a very high current spike through the diode while the capacitor charges to the supplied voltage.

When I look for diodes and see their peak pulse current, most of them are quite small compared to the "theoretical" infinite current you would have into a capacitor to charge it.


Say the ESR of a capacitor is 100 mOhms, and you charge it with a 20V source through the diode, the current spike in the diode will be about (20-.7)/.1 = 199 amps, and I have not found a normal diode that has this spec.

Yet I have seen, for example, a car battery with a diode after its positive terminal, to protect a reverse voltage connection, and then feed into a capacitive load (ceramic capacitors in front of a SMPS) and these diodes are fine.

Why do these diodes have small peak pulse currents, but can handle surge currents?
 
Engineering news on Phys.org
  • #2
In theory most power supplies will have an extremely high in-rush current to the filter capacitor. Rectifier diodes generally have a much higher Ifsm or "Surge Rating" than Iav or "Average Current Rating". For instance: http://www.diodes.com/datasheets/ds28010.pdf has an Iav of 10A, but an Ifsm of 600A. But we need to be clear here, this does NOT mean the diode can handle a 600A repetitive current, it simply means that the device can handle a brief spike of 600A (and this figure is a bit misleading, the actual maximum current is limited more by the junction temperature Tj).

A lot depends on what the diode is designed to do. In your example of a car battery with a protective diode, the diode does not need to be "fast" or have a "low forward voltage drop", it needs to have a high surge current and a sufficiently high Average Current to supply the SMPS. In the case of the output rectifier for an SMPS, other characteristics like reverse recovery time and low forward drop may be far more critical to efficient operation.

The key is selecting the diode that best fits the needs of your circuit.

Fish
 
  • #3
Thank you that is very helpful. I haven't seen a surge rating, but I wasn't looking for it either. I'll check some datasheets again.
 
  • #4
In the situation you describe, with ceramic capacitors of probably quite low values, the inrush current will flow for only a brief period. For instance, with R = 0.1 ohm and C = 10 micro farads we get a time constant RC = 1 microsecond. This is much shorter than the 8.3ms often quoted in a diode Ifsm rating.

The situation is of course different where very large capacitances are concerned (big electrolytics or even "supercapacitors"). In such cases, the inrush lasts far longer and so has to be treated more seriously. A deliberate series element or even an active "soft-start" arrangement may be necessary to keep the current to a tolerable level.
 

What causes max current spikes in diodes?

Max current spikes in diodes are caused by sudden changes in voltage or current. This can be due to switching transients, switching from AC to DC, or sudden changes in load.

Can max current spikes damage diodes?

Yes, max current spikes can damage diodes if they exceed the maximum current rating of the diode. This can lead to overheating and potentially cause the diode to fail.

How can max current spikes be prevented?

Max current spikes can be prevented by using a snubber circuit, which is a combination of a resistor and capacitor, to absorb the sudden changes in voltage or current. Additionally, using a diode with a higher current rating can also prevent damage from max current spikes.

What is the impact of max current spikes on circuit performance?

Max current spikes can cause a decrease in circuit performance by disrupting the flow of current and potentially damaging components. This can lead to malfunctions, voltage fluctuations, and overall instability in the circuit.

Can max current spikes occur in all types of diodes?

Yes, max current spikes can occur in all types of diodes. However, certain types of diodes, such as Zener diodes, have built-in features that can help mitigate the effects of max current spikes.

Suggested for: Max current spikes in diodes

Replies
2
Views
844
Replies
3
Views
170
Replies
11
Views
2K
Replies
15
Views
349
Replies
26
Views
1K
Replies
4
Views
815
Replies
19
Views
2K
Replies
2
Views
753
Replies
12
Views
2K
Back
Top