Protect Setup From High Voltage Peaks

This blog describes how a setup can be protected from overvoltage which occurs switching off a coil. Fast changing currents induce transient voltages in an inductive load which can be expressed by the formula:

V = – L (dI/dt)

L is inductance in Henry and dI/dt is the rate of change of current in Ampere per second.

The energy which is stored at the inductance at the moment of the interruption can be calculated with the following formula:

W(i) = 1/2*L*i^2

where W(i) is the energy in Joules and i is the instantaneous current in Ampere at the time of interruption.

Transient voltages can be much higher than DC bias voltages used by a developer.

Setup

As illustrative example, consider the circuit shown in figure 1. The inductance in the setup is 10 µH, the resistance R1 is 50 Ohm and limits the current through the inductance. Such a setup can behave like a spike generator. When the switch is closed current is flowing through the inductance. In this configuration high voltage transients can be measured when the switch is opened and the energy from the system is dissipated via the parasitic resistance R_parasitic.

 

 

Figure 1: schematic setup with small inductance of 10 µH and the real setup

 

Results

As an example, a +/-35 V peak transient can be injected into the system biased with 5 V when the system is switched off (switching inductance). In figure 2 the voltage transients are captured with an oscilloscope. The voltage oscillation occurs due to  parasitic capacitance in the range of ~50 Picofarad in the circuit. The oscillation frequency is in the range of 7.4 MHz. The ring down comes from internal resistance which absorbs the energy. The parasitic resistance R_parasitic can be estimated with around 30 Ohm. The voltage peaks can be harmful for sensitive electronics circuits.

Figure 2: transient voltage peaks measured with oscilloscope

 

Protection from Voltage Peaks

For avoiding high voltage peaks in the setup or at the measurement device, different protection measures can be taken. Note that the protection measures depend largely on the test setup and the nature of the stress conditions. Here is a small overview of some possible protection components:

  1. TVS – transient voltage suppression diode
  2. Resistor or fuse for current limitation
  3. Thyristor Surge Protection Devices
  4. Metal Oxide Varistors (MOV)
  5. Polymer ESD Devices

In this part the TVS and the resistor solution are discussed. (For further solutions we refer to the documentation in the resource part at the bottom of this blog.) The dimensioning of the TVS depends on several factors. TVS needs to absorb energy and should react fast on overvoltage peaks. It is a compromise between a robust TVS which can absorb energy and the capacitance of this component. It also has to be considered that the combination of the parasitic capacitance and the internal resistance act like a low pass filter which reduces the bandwidth of the system. For example if the setup consists of a resistor R2 of 100 Ohm and an assumed capacitance of 400 pF (measurement instrument capacitance and TVS capacitance) the bandwidth (-3dB) should be around 4 MHz.

Figure 3 shows an example with a bidirectional transient-voltage-suppression diode across the inductance without additional current limitation.

Figure 3: setup with transient-voltage-suppression diode

 

In the following figure 4 you see a setup with a 100 Ohm resistor which can also help to attenuate the high voltage transients.

Figure 4: setup with resistance R2 (or fuse) for current limitation

 

Figure 5 shows an example with a bidirectional transient-voltage-suppression diode across the inductance and a protection resistance which limits the current.

Figure 5: setup with transient-voltage-suppression diode and protection resistance

Conclusion

This blog shows how high voltage transients can occur already with small inductance  values as 10 µH and how your setup can possibly be protected from high voltage transients. If you have further questions please contact us on [email protected]

 

Resources:

Transient Voltage Protection, ON semicondutor, https://www.onsemi.com/pub/Collateral/TND335-D.PDF

How To Select Transient Voltage Suppressors: https://www.microsemi.com/document-portal/doc_view/14650-how-to-select-a-transient-voltage-suppressor

Parasitic Cpacitance in Transient Voltage Suppressors & Low Capacitance: https://www.microsemi.com/document-portal/doc_view/14607-micronote-110-parasitic-capacitance-in-tvs