Series and parallel connection of igbt
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IGBTs can be connected in series or parallel to achieve higher voltages or currents, respectively. When connected in series, the main challenge is maintaining equal voltage blocking among the IGBTs. Various passive and active techniques are used to address this, including snubber circuits, gate control methods, and voltage clamping. When connected in parallel, even current sharing must be ensured to prevent thermal runaway from uneven loading.
Series and parallel connection of igbt
- 1. Name: Mafaz Ahmed Series and Parallel Connection of IGBT: Connecting power semiconductor devices in series or parallel, they can higher voltages or higher currents respectively. Applications: high-voltage converters high-power traction and industrial drives FACT and HVDC transmission systems Series Connection: IGBT are connected in series to have high voltage rating and use in high voltage applications. The main problem of the series connection is unequal voltage sharing between devices in both transient and steady-state operations. The operating voltage of the series connected power IGBT is higher than the allowable individual device operating voltage and ideally must be shared equally between series connected devices. This voltage unbalance may exceed the individual device voltage rating and subsequent failure of this device causing final failure of entire series string of devices. In series connection of power IGBT, the main problem is to maintain the equal blocking voltages among the IGBTs. To resolve this problem we use the following techniques, Passive snubber circuits Active gate control methods Voltage clamping methods 1. Passive snubber circuits: Snubber capacitors minimizes the voltage unbalance but increases snubber power loss. It prevent IGBT from excess current and protect it. A resistor chain in parallel with series devices, is used for static sharing and a resistor-capacitor (RC) or resistor- capacitor diode (RCD) circuit is used in parallel with each series element for dynamic sharing as shown in figure below
- 2. Advantages of Snubber circuit: The components are large in size Withstand high voltages and currents, Disadvantages of Snubber circuit: Bulky and costly Switching characteristics are slowed down Device operating frequency has to be kept low. 2. Active gate control method: There are four Active gate control methods to control voltage unbalances, i. Voltage balance by enlarging Miller effect zone: By using this method collector-emitter voltages are balanced in the transient state. The main advantage of this method is the avoidance of passive components. Static balancing is done by connecting large resistors in parallel with each device. ii. Gate signal delay control: Difference between delays causes unbalance in voltage sharing, so in this method the gate signal is adjusted to control the transient and steady-state voltage unbalances. iii. Master-slave approach:
- 3. In this method one device is kept master and the collector- emitter voltage of other devices are control by that master device. But this method is quite sensitive to master voltage changes. So its control circuit is complex and costly. iv. Reference voltage method: In this there is a common reference voltage which is followed by all other devices. Common reference voltage is designer’s choice and does not depends on other devices. v. Voltage clamping by active gate control : In this method the collector-emitter voltage is controlled if it is greater than the reference voltage by applying a positive gate charge to the gate terminal. The main problems are high-power loss and higher dv/dt during the transient stage of the first device to switch off. vi. Active gate control by resistors, capacitors and a diode: In this method resistors are used to ensure steady state balancing while capacitors are used to ensure transient balancing. vii. Gate voltage reference clamping: In this method collector-emitter voltage is converted to a gate-voltage reference and the control circuit clamps the collector-emitter voltage at the desired level. viii. Gate balancing core method: This method is use for transient voltage balancing. In this method each gate drive circuit is coupled with a gate balancing core that induces gate currents whenever the switching signal is delayed. 3. Voltage clamping method: There are four voltage clamp methods, Zener diode clamping
- 4. Voltage clamping by Zener diodes and capacitors Voltage clamping by capacitors and diodes Multi-level voltage clamping Parallel Connection of IGBT: To increase the current rating we connect IGBT in parallel. But using this configuration we face some concerns that are, Steady-state (static) current sharing Dynamic current sharing during switching Thermal stability and maximum junction temperature External influence on sharing and parasitic oscillations. In parallel connection of IGBT we have to balance the current in all devices otherwise thermal runaway will occur, which in turn damages all parallel devices. We use the following technique to balance the current sharing.