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Posted on 01 December 2019

The Application-Specific Power Semiconductors Diodes

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One diode technology is not suited for all applications

Development trends in high voltage power devices continue to move towards high performance, application-specific devices, and higher levels of integration and advanced packaging. The demand for electricity is increasing and at the same time, the cost of power generation is also going up.

By Sampat Shekhawat, Fairchild Semiconductor

 

There is increasing pressure to reduce the harmful emission of gases. This is forcing equipment designers to increase efficiency and performance. It looks like even government agencies will set the new minimum efficiency limits. Different applications use different power topologies. In all these topologies, device parameters play vital role in circuit performance. This is generating the need to tune the device parameters and stray capacitances for each application. Almost all the power equipment used in the industry such as automotive and transportation, household and home entertainment, communication, power generation, military and aerospace, and alternate sources of renewable energy that one can think of, use these power devices. The performance, size and cost of this equipment also depends on these devices. This is driving the need for application-specific devices. This paper describes what kind of power devices are needed for different applications.

Introduction

Switch-mode power supplies (SMPS) and white goods are increasingly being designed with an active power factor control (PFC) input stage to meet international regulatory standards for harmonic content. Historically, power factor correction circuits have used a boost converter topology that combines a power switch and boost diode. The active boost PFC circuits use different topologies and need application-specific devices in-order to get the best performance. DC/DC power supplies encounter the power levels, some where from a few watts for battery-operated portable equipment, several hundred to several kilo-watts for home entertainment, computer and office equipment. Synchronous rectification is also becoming popular method to improve SMPS efficiency. Motor drives can use from a tenth of a Watt to few mega Watts. Rectifiers and inverters are designed for several hundred mega Watts of power levels used for transmission of electric power. All these systems need applicationspecific devices to optimize performance and reduce cost. The most popular power devices used are diodes, IGBTs, and MOSFETs will be discussed in these three different sections.

Diode

Rectifiers are one of the most common devices used in power electronics. Some of these common uses are input side rectification, secondary side rectification, AC switch, free wheeling diode (FRD) for inverters and DC/DC converters, boost diode for both discontinuous and continuous current mode PFC, etc. In all of these applications, diode selection is very important. Like any other semiconductors a diode is not an ideal device. If the diode is not selected or designed properly for the application, the diode parameters can increase the turn-on losses for the IGBT or MOSFET and even for the diode itself. Snappy diodes as shown in Figure 1 will increase the EMI in the circuit and also can destroy themselves as well as their co-pack IGBT switch in inverters (motor drive, UPS and solar inverter) during reverse recovery. The performance of P-i-N diodes has been continually improving as a result of new simulation tools to optimize structure. Maximizing operating frequency requires faster and faster diodes. In silicon technology, diodes have been optimized for different applications and these minority carrier silicon diodes are categorized as follows.

* Low VF with high QRR and TRR
* Moderate VF and moderate TRR
* High VF and low TRR
* Moderate VF but snappy diode

Figure 1 shows the comparison of reverse recovery and VF of some of these 600V different technology silicon diodes and also silicon carbide (SiC) Schottky diode too.

Reverse recovery mechanism of different diode technologies

Diode Switching Performance

Figure 2 shows a hard-switched (continuous conduction mode) boost PFC power supply circuit. The load is connected across VO. The control pulses are applied to the negative bus and gate of the MOSFET (Qb) through a gate driver circuit. In the inductor charging mode when Qb is switched on, the input current rises through the boost inductor (Lb) and the boost MOSFET. During this mode, the energy is stored in the boost inductor. In inductor discharging mode when the Qb is turned off, the stored energy in Lb is transferred to the load and output filter capacitor through Db. The Db conducts the current when Qb is off. The power dissipation during this time is due to the forward drop of Db. Just before diode is forced from a forward to reverse bias condition the intrinsic region of the P-i-N diode is flooded with minority carriers during forward conduction. These carriers must be removed from the P-i-N junction in order to support the reverse voltage. As Qb turns on again, the boost diode current IF, as shown in Figure 3, is transferred from the Db to Qb. The transfer of current begins when the gate voltage of the Qb equals the threshold voltage at (t = to). At time t = t1, all of the boost diode forward current has been diverted to Qb. The Db current decays at the rate diF/dt from time to to t1. The time duration from t0 to t1 (tr) is dependent upon the diF/dt. The turn-on energy loss E(ON1) in the boost switch during tr is given by following equation:

Equation 1

Boost PFC

 Higher tr means lower diF/dt, which will allow more minority carriers to recombine, hence less IRRM but that still increases E(ON1). At time t = t1, diode reverse recovery mode starts. An extra current starts to flow through the Cf capacitors, stray inductance, and boost diode during this mode. During ta, the diode is not yet able to support bulk capacitor (Cf) voltage. Therefore, Qb supports the bulk capacitor voltage. Hence, Qb is stressed due to the presence of both high voltage and high current. The results in significant power dissipation during ta for the boost switch. This Qb turn-on loss E(ON2) during ta is given by the following equation 2.

Equation 2

Transfer of current from boost diode to boost switch

If the IRRM is excessive, electrical stresses may exceed the SOA limit, and cause failure of Qb or Db. The higher IRRM also increases ripple current in the output filter capacitors CO. The IRRM and correspondingly ta of the diode should be lowered in order to reduce E(ON2). The IRRM can be reduced through the use of increased gate resistance (RG), which lowers diF/dt and increases tr and ta. This results in higher E(ON1) and E(ON2). This phenomenon is shown in figure 4. The reduction in IRRM gets overcompensated by increase in tr and ta. Conversely, as gate resistance is lowered, the resulting voltage overshoot and oscillations during the tb phase of diode recovery dictate the lower limit of RG. Reducing the gate resistance below this practical limit, which largely depends on the boost switch and boost diode combination, may increase reliability problem. Both IRRM and ta of the Fairchild StealthTM and StealthTM II diode have been reduced compared to competitors diodes for the same diF/dt. This lowers the turn-on loss of the Qb. Additionally, the soft recovery characteristics of the StealthTM design result in reduced oscillations during the tb phase. This property provides a measure of dynamic ruggedness, which is not present in snappy recovery diode designs. This is due to the fact that voltage overshoot present during the recovery of snappy diodes may be in excess of the rated blocking voltage of the boost diode or diode-IGBT co-pack for inverter applications. High voltage and current are present across the diode during tb, resulting in a higher dissipation. Large voltage spikes and ringing also may appear during this phase. This is caused by snappy recovery (high diRM/dt) of the diode and stray inductance in the current path. Snubber circuits are commonly employed to overcome voltage overshoot due to snappy recovery. Unfortunately, these components add cost to designs, take up valuable space and often contribute additional losses. Use of a soft recovery diode eliminates the need for a snubber because of reduced ringing and overshoot due to the lower diRM/dt during the tb phase. The Fairchild StealthTM diode has a softness ratio greater than one. For critical current mode or boundary current mode PFC, moderately low VF and moderately low IRRM diodes are ideal since there may be still some minority carriers may be left when boost switch is turned on.

Effect of gate resistance (Rg) on turn-on loss and diode recovery

As the operating frequencies (fs) of power supply designs continue to increase, diode performance must be optimized. Accordingly, fast and soft recovery diodes generally reduce overall losses by lowering the turn-on loss of boost switch and or co-pack IGBT. Additionally if the softness is increased too much then also IGBT turn-on loss (E(on3)) will increase as shown in equation 3.

Equation 3

Figure 4 shows that the increase in Rg increases E(ON3) even though IRRM is reduced but reduction of IRRM is overcompensated by increase in tb. Equation 4 shows total turn-on loss of the boost switch Qb:

Equation 4

The loss in the diode during to tb is shown in following equations 5:

Equation 5

This loss further increases as the ambient or diode junction temperature is increased. This causes further increase in stored charge and greater reverse recovery losses due to higher junction temperature.

Finally, diode leakage current is another important parameter to consider, especially at high temperature operation. The anode to cathode leakage current should be kept as low as possible in order for the power dissipation in the off state to be kept at a minimum. Forward recovery is also important. For hard switching application forward switching should be considered and diode should be designed or chosen with less forward recovery voltage and time.

Diode Selection

Another PFC known as discontinuous current mode (DCM) is used at very low power level where the diode recovery problem is not there and control is also simple. However the increased input pulse current increases EMI and the system can become unstable at light loads. This needs an oversized boost switch since R.M.S current is high due to higher peak current. Ripple current stresses the filter capacitor. There is no diode recovery problem so a low forward voltage (VF) drop diode can be used. Fairchild’s low VF UltrafastTM and moderate VF HyperfastTM diode will be good choice for this application. However low VF UltrafastTM diode has high QRR and TRR and can not be recommended for hard switched applications. The disadvantages of DCM PFC can be over come by CCM boost converter where boost diode and boost switch generally operate in the hard-switching mode. The drawback of hard switching is that the diode reverse recovery characteristics increase the switching device’s turn-on loss and the generated EMI. The diode’s reverse recovery characteristics describe how the device transitions from the forward conducting state to the reverse voltage blocking state. If the return of the reverse recovery current from IRRM to zero is too snappy as shown in figure 1, high voltage spikes and severe EMI are generated. Slowing down the switch turn-on rate increases the switch turn-on loss. Adding soft switching circuitry adds to circuit cost and complexity. However, with the use of Fairchild’s soft and low Qrr and Trr StealthTM diode, the diode snubber circuitry can be eliminated or reduced, the boost switch turn-on loss is reduced and CCM PFC can be implemented in the hard-switched mode up to 130 KHz .

For input side rectification UltrafastTM (low VF diode) diode provides the best performance because of low forward voltage drop. However for inverter (DC/AC) and DC/DC application HyperfastTM (moderately low VF diode) didoe technology is further optimized for the free wheeling diode (FRD) because the frequency of operation is not high. These two diode technologies can be used as FRD for high frequency soft switched applications too where co-pack diode turns on before its IGBT turns on. In such applications, the diode recovery problem is not there. The HyperfastTM diode has lower VF and higher QRR and TRR compared to StealthTM diode however it has higher VF and lower TRR and QRR compared to UltrafastTM diode, so it is good for hard switched medium frequency to high frequency applications. In some HV DC/DC applications where frequency (50 KHz to 130 KHz) of operation is high, StealthTM diode is recommended. As the rail voltage is increased >600V switching loss starts to dominate compared to conduction loss soft and faster recovery diode can be a better choice even at medium frequencies above 15 KHz operation.

For output side high frequency rectification, the diode can increase the turn-on loss of the primary side switches then HyperfastTM diodes or StealthTM diodes are recommended depending on operating frequency. One can chose one of these diodes depending on frequency of operation. The StealthTM diode can be used for high frequency and HyperfastTM diode can be used at low frequency for these applications since there is a trade of between forward voltage drop and diode recovery. This way that one can optimize the losses by selecting proper device. The following table shows the trade-off of high VF (Stealth), moderate VF (HyperfastTM) and low VF (UltrafastTM). These measurements have been taken at IF = 10A, TJ = 125°C, VR = 400V and di/dt = 400A/μSec.

Rectifier Selection Criteria

By using equation 4 and table 1 one can do trade-off and select these diodes depending on application.

SiC Schottky diode

Recently, high voltage (>300V) SiC Schottky diodes also known as near zero recovery (superior dynamic performance) diodes have been introduced by few companies and are gaining popularity. These diodes can be used for high end telecom and computer power supplies. The reverse recovery characteristic is shown in Figure 1 as is compared with P-i-N silicon diode. The only reverse current (IR) it carries is due to junction capacitance which is independent of temperature, forward current, etc. This current is very small as shown in Figure 4.

These diodes have the following advantages:

1. Majority Carrier Device so zero reverse recovery current.
2. Only small junction capacitance: the charging displacement current flows which acts like reverse recovery current but it is independent of temperature and diode forward current. This acts as QRR.
3. QRR is about 1/5th of the Silicon P-i-N diode.
4. SiC thermal conductivity is about two - three times than that of silicon.
5. Leakage current is low.
6. RSPON for SiC is about 1.7mOhm-cm2 compared to 73mOhmcm2 for Silicon @ 600V so Smaller foot-print.
7. Band gap of SiC is three times higher and break down field is ten times higher than silicon.
8. Much less EMI and reduced snubber size for boost and FRD.
9. Boost, Buck DC/DC and DC/AC inverter (FWD): Switch turn-on loss reduced.
10. SiC diode turn-off loss is less compared to Silicon P-i-N diode.
11. Higher operating temperature.
12. Reduces component count by converting soft switched topology to hard switched topology without effecting circuit performance.
13. Higher PFC efficiency and reduced heat sink size.
14. High frequency of operation so reduced boost inductor value for the PFC application hence simplified power circuit, high power density/compactness and high reliability.

But there is a trade-off between overall system cost and these advantages.

Figure 5 shows that the snappy diode has much higher diRM/dt during tb phase of the diode recovery and generates oscillation when it is going into blocking state. This oscillation can kill co-pack IGBT or even diode itself. SiC Schottky diode is ideal diode as a free wheeling diode for inverters used for motor drive, UPS, PV solar inverters, automotive grade inverters etc. Efficiency is very important for PV Solar Inverters and SiC Schottky diode can help to improve that. It also increases reliability because reverse recovery dv/dt is low compared to a snappy diode as shown in figure 4.

Reverse recovery comparison of Silicon and SiC Schottky diode

Conclusion

From above discussion, it is clear that one diode technology is not suited for all applications even though the name diode sounds very simple. For best performance, one has to choose the right diode since each applications has different needs. To meet the need of the customers, Fairchild has developed different types of diode technologies for a wide range of applications.

 

 

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