Tweet

Posted on 23 January 2019

Parasitic Inductances and Capacitances in Converters

 

 

 

 

 

 

 

To analyze the individual effects of parasitic inductances and capacitances and their interdependence in converters, it suffices to examine one commutation circuit. Figure 1 shows the commutation circuit of an IGBT converter with parasitic elements, consisting of DC link voltage Vd (corresponds to commutation voltage VK) and two IGBT switches with driver and antiparallel freewheeling diodes. The commutation voltage is impressed by the DC link capacitance Cd. The impressed current iL flows out of the commutation circuit.

Commutation circuit with parasitic inductive and capacitive elements

Figure 1. Commutation circuit with parasitic inductive and capacitive elements

The effects of parasitic elements and counter-measures

 Total commutation circuit inductance

In the commutation circuit with T1 and D2, the sum of L11 , L 61 , L31 , L 41 , L72 , L52, and L12 is effective. Similarly, the sum of L11 , L71 , L51 , L62 , L32 , L42, and L12 is effective in the commutation circuit with D1 and T2. During active turn-on of T1 or T2, the total commutation inductance becomes effective as turn-on relief, which will reduce turn-on power dissipation in T1 or T2.

However, during active turn-off of T1 and T2, as well as during reverse-recovery di/dt of D1 and D2, switching overvoltages are generated in the transistors and diodes due to high di/dt caused by the commutation circuit inductances. This increases turn-off power dissipation and voltage stress in the power semiconductors.

This effect is especially critical with regards to short circuits and overload. Moreover, undesired high-frequency oscillations in the range of some MHz may be generated in connection with parasitic capacitances.

In hard switching converters, it is therefore vital that inductances in the commutation circuit be reduced to a minimum. With the exception of L11 and L12, all inductances are generated within the modules and cannot be influenced by the user. In this respect, it is up to the manufacturers of power modules to keep on working on the minimization of internal inductances by improving module assembly technologies.

SEMIKRON datasheets, for example, indicate the internal inductances effective at the module output terminals (Example: SKM300GB12T4: LCE = typ. 15 nH; Example: SEMiX252GB126HDs: LCE = typ. 18 nH).

In the case of single switch modules (1 IGBT/MOSFET + 1 inverse diode), the connection of both modules has to be as low-inductive as possible in a converter phase or commutation circuit.

Particularly important here is that the DC link busbars are low-inductive. This applies to the busbars between the DC link capacitors, as well as to the connection of the power modules to the DC link. In relation to this, laminated busbar systems (tightly parallelled plate systems) adapted to the specific converter layout have become widely accepted in practice, achieving busbar inductances of up to 20...50 nH.

The effect of the remaining inductances L11 + L12 on the power semiconductors can still be reduced by connecting C, RC, or RCD-circuits directly to the DC link terminals of the power modules. In most cases, a simple C-circuit with film capacitors within the range of 0.1...2 μF is connected. In low voltage, high current applications, attenuated RC-circuits are preferred.

 Emitter/source inductances

 The elements L31 or L32 of the emitter/source inductances are effective in the power circuit as well as in the driver circuit of the transistors. Due to the fast di/dt of the transistor current, voltages will be induced that will have the effect of inverse feedback in the driver circuit (emitter/source inverse feedback). This, however, will decelerate the charging process of the gate-emitter capacitance during turn-on or the discharging process of the gate-emitter capacitance during turn-off, resulting in increased switching times and switching losses. The inverse feedback effect of the emitter may be utilized to limit the collector current di/dt in the case of short circuits near the modules. To minimize the inductances L31 and L32, power modules are equipped with separate emitter control terminals.

If several BOTTOM driver stages of a converter are supplied by a common operating voltage with negative DC link reference, the parasitic inductances between the ground connectors of the drivers and the negative potential of the DC link may cause undesired oscillations in the ground loops. This problem can be solved by way of HF stabilization of the driver operating voltage near to the output stage or, in high power converters, by way of separate supply voltage potentials for the BOTTOM driver stages.

 Inductances L21  and L22

The inductances L21 or L22 designate the inductance of the supply line between driver and transistor. Apart from increasing the impedance of the driver circuit, they may cause unwelcome oscillations with the input capacitance of the transistor. This may be remedied by using a short, low inductance connection between driver and transistor. Increasing the gate series resistance may dampen oscillations. This, however, will unintentionally cause an increase in transistor switching losses at the same time.

 Capacitances

The capacitances Cxx in Figure 1 stand for the intrinsic capacitances in the power semiconductors (voltage-dependent, non-linear) and cannot be influenced by the user. They indicate the minimum value for commutation capacitance CK and, generally speaking, cause a reduction in power dissipation during turn-off.

Additional power dissipation is generated during active turn-on due to the recharge process of the commutation capacitances. In high-frequency MOSFET applications (...>100 kHz...), in particular, these losses must be taken into account.

C11 and C12 result in the Miller effect, as well as dynamic dv/dt-feedback to the gate that slows down the switching process.

The circuit must be designed such as to prevent strong capacitive coupling between the inductive supply leads to gate and collector/drain as well as to gate and emitter/source outside the module, which is likely to cause additional high-frequency parasitic oscillations. This aspect is becoming increasingly important in fast high-voltage power MOSFET applications.

 

For more information, please read:

Electromagnetic Interference in Converters (EMI)

Fuse Placement in Typical Converter Circuits

Rectifier Circuits

More Efficiency for 3-Level Inverters

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 197 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.