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A novel technique to reduce the reverse recovery charge of a power diode

N.Y.A Shammas, S.Eio STAFFORDSHIRE UNIVERSITY

P.O. Box 334, Beaconside Stafford, United Kingdom, ST16 9DG

Tel.: +44 / (0) – 1785353265. E-Mail: n.y.a.shammas@Staffs.ac.uk, s.eio@Staffs.ac.uk

URL: http://www.Staffs.ac.uk

Keywords Power semiconductor Device, Fast recovery diodes, Device characterisation

Abstract

Power diodes are required to have short reverse recovery time, soft recovery, high breakdown voltage and a low forward voltage drop at the rated forward current. Manufactures introduce recombination centres in the device to achieve these parameters; however, a trade-off between the on-state losses and breakdown voltages, and a trade-off between the on-state losses and switching speeds is still significant.

In this paper, a post manufacturing technique that reduces the reverse recovery charge of a power diode is proposed. The reverse recovery characteristic of the power diode was simulated by reconstructing a test circuit similar to the one previously published. To implement the technique, a current injection circuit, which injects an additional current from a pre-charged capacitor into the power diode prior to current zero, was added into the test circuit with results indicating a significant reduction in the power diode’s reverse recovery charge. This ‘helps’ to cancel the reverse current due to the stored electronic charge in the wide drift region and therefore prevents the device from conducting large reverse current. Nevertheless, the technique requires precise timing for injecting the appropriate forward current pulse seeing that premature injection will increase the forward current, and belated injection will either lengthen the power diode’s forward conduction period or switch the already turned off diode into its low impedance state. Two methods were developed to detect the falling anode current. The first method uses a digital signal processing (DSP) kit to predict the time where the falling anode current reaches zero based on its average gradient. The second method uses an analogue circuit to trigger the current injection circuit at a reference current level. Of the two methods, the DSP kit is limited by the processor’s response time; and the analogue circuit is restricted by its inability of predicting current zero. Therefore, the limiting factor of this technique is the electronic delay in its components.

Introduction

With the introduction of fast MOS-Bipolar power devices, the operating frequency in power electronic systems has increased. This has lead to the reduction in size of passive components and the increase in system efficiency. These MOS-Bipolar power devices produce low on-state losses, low switching losses and have high current densities. Power diodes are frequently used with these fast MOS-Bipolar devices in most power electronic circuit as freewheeling and snubber components, and are required to meet the demands of short reverse recovery time, soft recovery, high breakdown voltage and a low forward voltage drop at the rated forward current.

The power diode has a wide lightly doped layer to support the reverse breakdown voltage. This layer adds significant ohmic resistance to the power diode during its forward conduction state, and stores the electronic charge. This result in the reverse recovery characteristics and device designers introduce recombination centres in this layer to reduce the stored charge. [1, 2] However, with significant

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advancement in the development in power diodes, designers of power diodes are still struggling with trade-offs between the on-state losses and breakdown voltages, and a trade-off between the on-state losses and switching speeds.

The factors that determine the reverse recovery characteristics of a power diode can be separated into two main areas, internal device parameters and external circuit parameters. The internal device parameters include the device geometry, doping profile, length and resistivity of the drift region, and minority carrier lifetime. [3] The external circuit parameters govern the circuit operation and layout such as stray inductance, applied reverse voltage, commutating di/dt, forward current, and junction temperature. [4] External circuit parameters can be controlled in the design of power systems, but internal device factors can only be improved during its manufacturing phase. The application of this work focuses on testing and developing the proposed technique to reduce the reverse recovery charge of a given power diode.

Theory

Figure 1 shows the anode current (i) and voltage drop (V) across the power diode during its turn-off transient, staring at 1t and ending at 3t . At 1t the forward current is reduced and the current falls to zero at the rate di/dt which is determined by the external circuit inductance and the applied reverse voltage. The anode current falls to zero and turns negative during its storage time 2t ; this is because of stored electronic charge in the drift region. [5, 6]

Fig. 1: Turn-off transient

The reverse recovery charge Qrr can be affected by the internal device parameters and external operating conditions. [4] External factors govern the circuit operation and layout such as stray inductance, applied voltage, commutating di/dt, forward current, and junction temperature. These factors can be improved in the design of power system circuitry. However, internal device parameter that includes the device geometry, doping profile, and minority carrier-lifetime value can only be improved during its manufacturing phase.

Carrier lifetime killing techniques [7, 8] reduce the stored charges in the drift region of the power diode, reducing the reverse recovery time. However, long carrier lifetime are required in high voltage devices to maintain low ON-state losses and reducing the carrier lifetime results in trade-offs between its conduction losses and dynamic losses. Doping metals is a carrier lifetime killing technique [1, 8] which controls the minority carrier lifetimes by introducing recombination centres into the device; this reduces the stored charge in the drift region. Gold was the preferred metal used because it provides optimal recombination centre [9], however later research indicates that gold-doped structures are more

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likely to produce oscillations during switching transients. This caused the move from Gold to Platinum. [10, 11, 12] Doping metals is a proven technique to improving the switching transients of the device but it is also known to increase the conduction losses.

High-energy irradiation is the second technique of carrier-lifetime killing where high-energy irradiation is used to introduce recombination centres in the device. Electron, Proton and Gamma irradiation were used to perform controlled damage to the Silicon Crystal by irradiation. [13, 14, 15] This creates controlled defects, and displaces atoms from its normal position which loses their energy in interactions with the lattice atoms, creating vacancies, deviancies, impurity-vacancy pairs, interstitials and impurity-vacancy-interstitial complexes that all introduces recombination centres to reduce the stored charge in the drift region. High-energy irradiation is an effective method to introduce recombination centres in a device, however defects created can be unstable and may be anneal out at working temperatures of the device. This can increase the conduction losses, leakage currents, and reduced breakdown voltage capabilities of the device. Proton and alpha irradiation are costly to perform as a vacuum environment is required for this technique. [16, 17, 18]

The proposed technique

This technique benefit power application designers by providing them with the flexibility of using power diodes with low conduction losses, and implement this technique to improve and ‘fine-tune’ the device reverse recovery characteristics. This combination can provide a near ‘ideal’ situation where an application can achieve both low switching and low conduction losses, concurrently.

The test circuit

A test circuit similar to the RAMP test circuit previously published [2, 19] was reconstructed. In figure 2, the diode under test (DUT) was subjected to a steady state forward current followed by the application of reverse bias voltage. An IRGPC50S Insulated Gated Bipolar Transistor, IGBT was used to control the circuit’s forward conduction and bringing the 1N5401 power diode (DUT) into its low impedance state with a current pulse of 1A and a period of 2ms. At the end of the pulse, the reverse circuit controlled by a Thyristor 30TPS12 SCR, triggers and applies a negative voltage, forcing the diode to its reverse recovery state. This circuit is capable of demonstrating the effects of circuit parameters and operating conditions of the reverse recovery. The ability to vary the commutating di/dt by changing the reverse voltage applied across the diode allows the power diode to produce both soft recovery, and snappy reverse recovery characteristics.

Two high voltage, low inductance capacitors (C1, C2) with ratings of 1.4kVdc 20nH, 200uF capacitors were charged to provide variable high voltage testing up to 1kV. The voltage across the capacitors was measured with 1.2kV digital multi-meters, the anode current of the DUT was obtained with a 1A:0.1mA current transducer and the voltage across the device was obtained with a Tektronix high voltage probe of 120MHz bandwidth and 1.5kV, both are connected to a Tektronix’s TDS3054B 4 channel digital phosphor oscilloscope.

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Fig. 2: RAMP test circuit

Triggering sequence

The triggering sequence of the IGBT and the thyristor are shown in figure 3. This triggering sequence is controlled by an electronic circuit by the activation of a mechanical switch. A 15Vdc gate pulse label as channel II in the figure triggers the IGBT into conduction for a period of 2ms. This provides sufficient time for conduction modulation to take place in the D.U.T., and not too long a period for the D.U.T. to self-heat during high power test.

Soon after the 2ms period of the IGBT gate pulse, the thyristor is triggered by a short pulse as shown in figure 3 (channel I) allowing the negative voltage to appear fully across the circuit, which will be blocked by the power diode when it recovers.

Fig. 3: Gate pulse

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Reverse recovery characteristics

Figure 4 shows the reverse recovery characteristic of the DUT in the test circuit and the peak reverse recovery current rrI is approximately -1.1A.

Fig. 4: Diode’s current waveform obtained practical circuit

Implementation of the proposed technique

Fig. 5: Test circuit with the implementation of the proposed technique

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The circuit in figure 5 is similar to the circuit in figure 2. However in figure 5, a current injection circuit (highlighted with dotted lines) is added into the test circuit. This current injection circuit is required for implementing the proposed technique. When the DUT was abruptly reverse biased, the stored charge in the drift region of the DUT prevents the junction from recovering to its reverse blocking state. Therefore, prior to the anode current falling to zero, the current injection circuit injects an additional current from a charged external DC source (C3) that is proportional to the negative peak current into the DUT. This additional current ‘helps’ the power diode to cancel the reverse current and prevents the device from conducting large reverse current.

Fig. 6: Anode current waveform at reverse recovery

Figure 6 compares two anode current waveforms obtained from the DUT. The first anode current waveform label ‘original’ is the anode current of the DUT (similar to figure 4) obtained using the test circuit without the addition of the current injection circuit. The second anode current waveform shown in figure 6, label ‘CI method’ is the anode current waveform of the DUT obtained by repeating the test using the same test circuit but with the addition of the current injection circuit. This figure shows a significant reduction in the reverse recovery charge and peak reverse recovery current in the waveform label as ‘CI method’ when compared with the anode current waveform label as ‘Original’.

Detection and Triggering

Two methods were used to detect the falling anode current required to trigger the current injection circuit. The first method was developed with a digital signal processing development kit and is able to calculate the average current rate of fall and predict the time where the anode current reaches zero. This provides the flexibility to ‘fine-tune’ the triggering time of the current injection circuit based on the predicted time. However this method is limited by the processing speed of the digital signal processing kit and therefore is only suitable for low di/dt rating applications.

For high di/dt ratings, the second method uses a high speed analogue circuit that include a high speed operational amplifier. This circuit triggers the current injection circuit when the falling anode current matches a reference current level. The advantage of this method is the reduction in electronic delay due to few components used. However it lacks the ability to predict the time where the anode current reaches zero and results in a short peak at the anode current, before falling to zero.

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Conclusion When the power diode is reverse biased from its forward conduction to blocking state, the stored charge in its wide depletion region prevents the device from recovering to its reverse blocking state and resulting in the reverse recovery characteristics and reverse recovery charge. Reducing the reverse recovery charge indirectly reduce the limitations on the highest operating frequency. To improve the device reverse recovery characteristics, manufactures uses carrier lifetime control techniques during the device manufacturing phase to introduce recombination centers. However, this result in a trade-off between the on-state losses and breakdown voltages, and a trade-offs between the on-state losses and switching speeds. The proposed post manufacturing technique uses a current injection circuit to reduce the reverse recovery charge of the power diode. This provide application designers the flexibility to use power diodes with low conduction losses, and implement the proposed technique into the application, to reduce the reverse recovery charge, thus achieving near ‘ideal’ power diode operation. The limitation of this technique is its critical current injection time where premature injection will increase the period of conduction, and belated injection will cause unwanted forward current. Investigation to improve the detection and triggering of this technique and the implementation of this technique on power thyristor and IGBT are being considered.

References

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