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Posted on 30 April 2019

Compact and Powerful 600V Half Bridge Driver ICs for Consumer Electronics and Home Appliances

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Developers of consumer electronics and home appliances strive continuously for higher efficiency of applications and smaller form factors. One area of interest in power supply design is that the switching behaviour and power losses of new power MOSFETs, such as the latest generations of CoolMOS™ with dramatically reduced gate charges, can be optimized by dedicated driver ICs.

By Wolfgang Frank, Oliver Hellmund and Viktor Boguszewicz, Infineon Technologies AG, Neubiberg, Germany

A new family of half bridge gate driver IC supports both design goals. With its monolithic integrated ultrafast bootstrap function this new generation 2EDL EiceDRIVER™ is a benchmark for driver ICs in the market with more than 2A output current. The new 600V gate driver IC family currently contains seven devices, with output currents of 0.5A and 2.3A, for applications with either IGBTs or MOSFETs. With the new 2EDL driver ICs, Infineon also introduces a new device segment; EiceDRIVER™ C (Compact) for consumer electronics and home appliances. This article discusses the driver’s operation range and the properties of the bootstrap function in terms of dependency on temperature and IC supply.

Introduction

The new 2EDL half bridge driver ICs represent a new class of gate drivers with integrated bootstrap function for the high side supply. The reasons there are few devices of this class on market include a rather high voltage drop at low duty cycles and the additional high power dissipation in the IC at high switching frequencies. Existing devices are typically limited to applications such as consumer drives. They have 600V blocking voltage, which is the low power drives market. Other half bridge driver ICs, which do not have an integrated bootstrap diode are used in low end switch mode power supply (SMPS) applications. Since there is no bootstrap function integrated into the IC, these products have a slightly better temperature budget due to less power dissipation.

However, the advantages of a powerful integrated bootstrap function are striking: simpler layout, less PCB space and better component placement in respect of distance to the gate terminal of the power transistor. This keeps also the EMI low and optimizes the switching performance, hence the switching losses.

For these reasons, Infineon developed the new concept of a half bridge Gate driver IC that fully supports the design considerations of consumer electronic equipment including SMPS and computing, and home appliance drive systems.

The new half bridge driver IC family 2EDL is designed to support all mega trends in low power drives designs, such as ease of use and short bill of materials while simultaneously offering a high number of features.

Technology

Thin-Film-SOI (Silicon-On-Insulator) technology is an advanced technique for MOS/CMOS fabrications. Based on conventional bulk process the SOI technology uses an insulator called buried oxide underneath the active device layer, as shown in Figure 1.

Silicon on insulator transistor

The lateral insulation of elements inside the silicon film is achieved by a simple local oxidation (LOCOS) process. In this way all active device regions are fully insulated from each other. Thus, there is no need for CMOS-wells to prevent the “latch-up” effect. Additionally, leakage current and junction capacitances are reduced significantly. The small size of PN-junctions inside the thin silicon film leads also to higher switching speed.

Thin film SOI process

Different devices inside the individual silicon regions can be implemented as shown in Table 1. The presented technology contains 600V devices like level-shift transistor and high-voltage bootstrap diode, which are also realized in the thin silicon film. The 600V voltage ability is achieved by special junction termination structures, which allow monolithic realization of circuits like half or full bridge drivers for 600V applications.

Device family

The 2EDL EiceDRIVER™ Compact family covers the full range of peak output currents from 0.5A to 2.3A. The 0.5A device is available in DSO8 and DSO14 package, while the 2.3A device is available in DSO14. All packages are RoHS compliant, green and halogen-free. The device in DSO8 package provides a floating high side section with a limited functional set and feature set.

The 2EDL05I06BF is well-suited for SMPS. It does not have a dead time and interlock function, so that both the high side output and the low side output can be activated simultaneously.

A full set of protection functions and features, such as an enable function, a failure indication, separate return path for the gate current (power ground) incl. an overcurrent protection (OCP), is realised in two parts with high output current of 2.3A. Therefore, all applications with higher integration and safety requirements can be addressed.

Table of individual function set realized in new half bridge concept

Bootstrap diode

The integrated bootstrap function is typically realized by integrated high voltage MOSFET structures, as indicated on the left side of Fig. 2. The MOSFET structures are turned on and off in phase with the LS transistor. This is a crucial point, because the driver IC is neither aware of the delay times of the power transistors nor of the power factor of the motor. Thus, the control of the bootstrap FET must consider this by additional bootstrap delays. These delays reduce the available time for bootstrapping, so that the bootstrap voltage is further reduced.

Bootstrap circuit of half bridge

Another drawback of using a MOSFET for bootstrapping is the temperature dependency of MOSFET over temperature. Usually, MOSFETs double their RDS(on) – value when the junction temperature increases by 100°C. This means, that the above mentioned situation gets worse. The higher RDS(on) also causes more power dissipation inside the driver IC and limits the thermal safe operation area in respect to switching frequency and gate charge. It can be seen in Fig. 3, that the bootstrap diode is superior to existing bootstrap functions as soon as the diode forward characteristic is above the MOSFET characteristic. This is the case for a forward current of approx. 5 mA – 10 mA under elevated temperature.

Bootstrap diode forward characteristic compared with MOSFET

The effects of the output characteristics are visible in a diagram showing the nominal voltage reduction of the bootstrap capacitor voltage in respect to the supply voltage versus the duty cycle. As an example, a single half bridge configuration is used as a representative of a SMPS topology. A small duty cycle of the low side transistor or diode leads to an uncompleted recharging of the bootstrap capacitor CBS, as seen on the right side of Figure 2. As a consequence, the bootstrap voltage decreases until a new steady state operation is reached in respect to the supply voltage of the driver IC. Figure 4 shows the diagram for operating conditions of switching frequency fp = 20kHz and a bootstrap capacitor of CBS = 22μF.

Voltage drop vs duty cycle of buck converter

The left part in Figure 4 shows the conditions at a junction temperature of Tj = 25°C and the right part shows the same parameter at Tj = 125°C. The proposed driver IC concept of EiceDRIVER™ 2EDL realizes a bootstrap resistance of RBS = 30Ω at a junction temperature of Tj = 25°C, while other concepts have RBS = 125Ω or RBS = 200Ω. For reasons of simplicity, it is assumed that the bipolar drift region resistance of a pn-diode doubles its resistance every 100°C. Note that the size of the bootstrap capacitor does not influence the diagrams of Figure 4. It only influences the transition phase from one bias point to another.

The influence of the low resistance of the new EiceDRIVER™ 2EDL is significant. It is easy to see that the new driver IC concept is much more stable against high junction temperatures compared to standard parts. The usable duty cycle range can go down to 1% with the new 2EDL driver concept, without coming into an undervoltage lockout area.

Other driver ICs cannot serve duty cycle ranges below 4% (Rbs = 125Ω) or 7% (Rbs = 200Ω). This means that many applications which require operating at low duty cycles cannot use these ICs. This is the case for SMPS operating in hard switching under high load condition or drive systems, which operate with high torque at low speed in field oriented control. In these examples the control is either in steady state or quasi steady state operation in the critical duty cycle range.

Asymmetric undervoltage lockout

The EiceDRIVER™ 2EDL family is supported with dedicated designs for operation with IGBTs. Other on market driver IC’s only support MOSFET transistors in respect with the undervoltage lockout (UVLO) function. Gate threshold voltage of MOSFETs (e.g. 3V) compared with IGBTs (e.g. 4.6V – 5V) allow operation of the MOSFETs with much lower gate voltages. This is represented as well in the UVLO levels of the driver IC. On the other hand, it is dangerous to operate IGBTs using driver ICs that provide MOSFET UVLO levels, because the MOSFET UVLO levels are so low that the IGBT partially or fully desaturates. This effect causes highly increased losses and temporary operation in this mode can cause severe damage at the IGBT. It is therefore essential to operate IGBTs only with driver ICs that provide suitable UVLO levels for the IGBT.

An important aspect of the design of undervoltage lockout (UVLO) levels is the support of integrated bootstrap diodes. They have a relatively high forward voltage drop which contributes to the bootstrap voltage reduction compared to the supply voltage VDD of the IC seen in Fig. 4. The static bootstrap voltage vBS is in total

\begin{equation} v_{BS} = V_{DD} - v_{DBS} - v_{CE} \end{equation}

where vCE is the transistor voltage of the low side transistor in a half bridge configuration. Please note that this converts into the diode forward voltage when operating the low side diode.

It is easy to see that the high side output HO generates a smaller voltage, because the voltage vBS at the IC terminals VB and VS is reduced by the values vDBS and vCE. However, it is favourable to activate the UVLO for the high side supply VBS at a similar point of time as the low side supply VDD in order to prevent insufficient supply of the high side gate. Therefore the low side UVLO is activated at approx. 1V higher levels as the high side UVLO function. It also allows shifting the shut down levels VCCUV- of the low side towards slightly higher value. This can be achieved by implementing an asymmetric UVLO which considers different thresholds for high side and low side as shown in Table 3.

Asymmetric high side and low side UVLO levels

Other helpful features

UVLO filter

It is often very difficult for design engineers to realise a good trade-off with system boundaries such as geometry and component placement in order to achieve the best performance. An important item is the placement of the blocking capacitors for the supply voltages VDD and VBS. The above mentioned restrictions lead often to some distance between the IC and the blocking capacitor. This can cause inductive voltage drops at the pin VDD or VB during the turnon transient with the consequence of undesired UVLO events according to Figure 5.

Small filters for UVLO improve the noise robustness of the 2EDL family

A filter to suppress such short time voltage drops makes the IC more robust against noise on the supply lines. This is shown as an example on the very left side of Figure 5. The filter time is approximately 1.5μs, which is enough to filter all regular transients in respect with switching transients. However, the IC will control an UVLO and therefore the turn-off of the correlated outputs if the voltage drop is lower than 7.5V, which is given in the middle of Figure 5. The IC will also shut down its outputs, if the voltage drop is longer than the filter time.

Active shut down

The active shut down function is activated as soon as the supply voltage either on VDD or VBS reaches 3V – 4V. Leakage currents which may flow via the gate-collector path can potentially turn-on the device. The proposed concept clamps any leakage current of IGBT gate or MOSFET gate to the supply pin. The sink transistor inside the driver IC is activated when the supply voltage exceeds approximately 3V. The sink transistor is operated in its linear region and will clamp the gate. Small leakage currents in the range of μA can be clamped very efficiently as shown in Figure 6. As a consequence, gate-emitter resistors can be omitted.

Output sections keep IGBT below its threshold voltage even without supply voltage

Conclusion

Infineon introduced its 2EDL half bridge gate driver IC family as part of the segment EiceDRIVER™ Compact. This new driver IC concept for half bridge configurations is proposed for consumer electronic equipment including SMPS and computing, and home appliance drive systems. It is optimised for the two mostly used transistor technologies, IGBT and MOSFET. The integrated bootstrap diode has a very low ohmic series resistance and hence enables the widest operating range in respect with controlled duty cycles. The bootstrap diode dissipates minimal power inside the IC. This makes this 2EDL driver ICs a benchmark in the market. Further functions, such as the asymmetric undervoltage lockout or the active shut down support in particular help IGBTs overcome restrictions in terms of component placement and performance.

 

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