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MOSFETs with a fast body diode - Using the LLC topology and FREDFETs to boost efficiency

  Rutronik

In view of the global energy crisis, the current focus of electronic equipment is to achieve high power combined with lower energy consumption. Consequently, many electronics companies are increasing their standards for the efficiency of numerous products in their specifications. These can scarcely be met using conventional hard-switch converters. Developers of power supply units have therefore turned to soft-switch topologies such as the LLC resonant converter in order to improve the efficiency and enable operation at a higher frequency. However, there are a couple of aspects that must be considered.

A resonant LLC half bridge guarantees zero voltage switching (ZVS) in the entire switching device before it switches on (or zero current when it switches off). As a result, energy losses can be avoided by overlaying the switching current and voltage during each transition. With this circuitry, switching losses can also be kept low at high frequencies and the size of reactive components therefore reduced. Of course, lower losses also allow the use of smaller heat sinks. The zero voltage condition results from the intrinsic conduction of the MOSFET body diode. During extremely fast load changes, the MOSFET can transition from a zero-voltage to a zero-current switching condition. In this case, high dv/dt values could switch the intrinsic bipolar transistor to the conductive state, which generally results in the destruction of the MOSFET.

LLC topology

The basic circuit of a half bridge in LLC topology comprises two mechanical switches: the high-side mechanical switch (Q1) and the lowside mechanical switch (Q2). They are connected by an inductor Lr and a capacitor Cr to the transformer (see Image 1). The mechanical switches are bridged by their intrinsic body diodes (D1 and D2) and the intrinsic capacitive output resistance (C1 and C2). To clarify their role in the general operating principle, these are shown separately in Image 1. In addition, another inductor, Lm, can be seen. This is the leakage inductance of the transformer, which plays an important role in the LLC topology.

If one assumes that the primary inductance value Lm of the transformer is so high that it has no effect on the resonance network, then the converter shown in the figure above acts as a series resonant converter.

In a resonant cell, the maximum amplification is achieved if the frequency of the input signal (fi) is equal to the resonance frequency (fr), that is, if the LC impedance is equal to zero. The converter used operates in a frequency range that is limited by two specific resonance frequency values. These values are dependent on the circuit. The LLC controller sets the switching frequency (fs) of the MOSFET equal to the resonance frequency of the switch to guarantee the valuable advantage of the resonance.

During a load change, the resonance frequency changes from a minimum value (fr2) to a maximum value (fr1): See  image 9. For fs ≥ fr1 , LLC acts as a RC series resonant circuit. This operating principle applies at high load, that is, if Lm is faced with a low impedance. Conversely, for fs ≤ fr2, LLC acts as an RC parallel resonant circuit, which is the case at low load. This does not normally occur because the system would then operate in ZCS (zero current switching) mode. If the frequency fi is in the range fr2 <fi <fr1, these two operating principles are combined.

If the amplification of the resonant cell is displayed in graphic form, we get the curves shown in Image 3. This shows how the curve shape changes depending on the quality Q.

The operating range of the LLC resonant converter is limited by the maximum amplification. Bear in mind that the maximum voltage amplification is not available at fr1 or fr2. In fact, the frequency at which maximum amplification is achieved is between fr2 and fr1. As the quality Q reduces - that is, as the load decreases - this maximum amplification frequency shifts to fr2, and a higher maximum amplification is achieved. With increasing quality Q - that is, increasing load - the maximum amplification frequency shifts by contrast to fr1, and maximum amplification decreases. Therefore, full load is the least favorable case for a resonance network.

With regard to the MOSFET, as mentioned above, there is a key advantage of resonant converters with LLC when it comes to softswitching MOSFETs, while the sinusoidal output current reduces the emitted interference (EMC) for the overall system.

Image 4 illustrates typical waveforms for an LLC converter. It also clearly shows that the drain current Ids1 oscillates into the negative before becoming positive. The negative current value signifies that the body diode is conducting. In this phase, the drain-source voltage of the MOSFET is very low because it is dependent on the drop-out voltage at the diode. If the MOSFET switches while the conductivity of the body diode is practically zero, a transition to ZVS takes place, which reduces switching losses. As a result, the size of the heat sink can be reduced, thereby boosting the efficiency of the system.

If the switching frequency fs of the MOSFET is less than fr1, the current at the converter takes on a different shape. If this continues long enough to produce an intermittent current at the output diodes, the current on the primary side deviates from the sinusoidal waveform.

Furthermore, if the intrinsic output capacities C1 and C2 of the MOSFET have a value comparable to Cr, the resonance frequency fr is also dependent on the component. To avoid this and to make the fr value independent of the components used, it is important to select a Cr value greater than C1 and C2 at the design stage.

Freewheeling and the ZVS condition

Analysis of the equations relating to the resonant frequencies shows that the input impedance of the resonance network is inductive above the maximum amplification frequency and that the input current of the resonance network (Ip) remains below the voltage (Vd) applied to the resonance network. Below the maximum amplification frequency, the input impedance of the resonance networks is capacitive by contrast and Ip is greater than Vd.

During operation in the capacitive range, a polarity reversal of the body diode in voltage terms takes place during the switching operation, while the body diode is still carrying current. This subjects the MOSFET to a very high risk of failure. As highlighted in the yellow circle (Image 6), the reverse recovery time (trr) of the internal body diode is extremely important.

In accordance with this point, during a transition from low to high load (see Image 8), the control circuit (LLC controller) should be able to cause the MOSFET to switch to ZVS mode and to a positive shutdown current range. If this is not guaranteed, the MOSFET could operate in a hazardous range.

At a constant low load, the system operates close to the lower resonance frequency fr2. In this case, ZVS mode and a positive shutdown drain current are guaranteed. After the load change (from low to high), the switching frequency should follow the new resonance frequency. If this is not the case (as shown by the green line in Image 8), the system status is in range 3 (ZCS range). This means that ZVS mode and a positive shutdown drain current are not available. If the MOSFET is switched off, current therefore also flows through its body diode. If one analyzes the transition from low to high load in the amplification diagram, the following can be established:

The black dotted line plots the ideal course during the transition, whereas the green line corresponds to the actual course. As one can see, the system operates in the ZCS range during the transition from low to high load. The performance of the internal body diode then becomes extremely important. Therefore, the trend with the new LLC concept is moving toward the use of circuit breakers with a very low recovery time in the body diode.

Evaluation and reference boards

In order to develop a switch-mode power supply, we recommend gathering experience using an evaluation or reference board. They can also be used to test MOSFETs with a fast body diode and evaluate their advantages. These are also available for LLC topologies in different variants from Rutronik.

The STEVAL-ISA132V1 evaluation board can supply a continuous output of 170W (VIN = 190V to 264V AC, VOUT = 24V) and a peak output of over 300W for a limited time. Its architecture is based on a single-stage LLC resonant converter without PFC and the L6699 resonant controller. This incorporates some innovative functions such as self-adjusting, adaptable dead time, anticapacitive protection of the operating mode and a proprietary Safe-Start process that prevents hard switching during start-up.

The EVLSTNRG-170W evaluation board opens up the possibility of gaining experience using digital control of both the PFC stage as well as the LLC converter based on the STNRG388A digital controller. In this case, the upstream PFC stage operates in what is called "Enhanced Constant ON Time" mode (DCM-CCM boundary), and the LLC converter in "time- FREDFETs (Fast Recovery Epitaxial Diode Fet) are MOSFETs with a fast body diode. Siemens introduced them in the late 1980s as better alternatives to standard MOSFETs for hard-switch bridge topologies (half bridge, full bridge, 3-phase bridge) and especially for frequency converters. Common high-voltage MOSFETs are scarcely suitable for bridge circuits when operated at higher switching frequencies because their body diodes have relatively long disable delay times. Rapid switching generates high reverse currents, which cause high losses, particularly in the opposing mechanical switch. Added to this are high interference voltages, which must be reduced again with the help of additional filtering. Meanwhile, IGBTs (Insulated Gate Bipolar Transistor) have largely eclipsed FREDFETs in frequency converters, motor controllers and similar because they are often the better option in terms of price and in technical terms. Despite this, FREDFETs continue to play an important role in specific switch-mode power supply topologies. One of these, which is becoming increasingly popular, is the 'LLC converter'. Ideally, LLC converters operate in the resonant mode, which is also referred to as 'soft switching'. For this topology, STMicroelectronics offers MOSFETs with characteristics that are precisely optimized for this purpose as well as LLC controllers (analog and digital), including integrated 600V half-bridge drivers. Background of FREDFETs shift-controlled" mode (TSC). The board is designed for a continuous output of up to 170W. The application supports multiple output voltages: 24V (6A) for the main application, 12V (2A) e.g. for a controller and 5V (2A) for stand-by operation (always-on).

Another interesting board for smaller outputs is the EVLCMB1-90WADP. This is a 19V/90W converter that is specially designed for the typical specification of an AC/DC adapter for laptops and notebooks. Naturally, the board can also serve as the basis for further applications provided that the output voltage is adjusted accordingly in the target design. It has a wide mains input range (90V to 264V AC at a frequency of 45 to 65Hz) and very low power consumption at low load.

Once again, its architecture is based on a two-stage approach: a transition mode PFC preregulator and a downstream LLC halfbridge resonant converter. Both controllers, for the PFC stage as well as that for the LLC converter, are integrated in the STCMB1 Combo IC.

Background of FREDFETs

FREDFETs (Fast Recovery Epitaxial Diode Fet) are MOSFETs with a fast body diode. Siemens introduced them in the late 1980s as better alternatives to standard MOSFETs for hard-switch bridge topologies (half bridge, full bridge, 3-phase bridge) and especially for frequency converters. Common high-voltage MOSFETs are scarcely suitable for bridge circuits when operated at higher switching frequencies because their body diodes have relatively long disable delay times. Rapid switching generates high reverse currents, which cause high losses, particularly in the opposing mechanical switch. Added to this are high interference voltages, which must be reduced again with the help of additional filtering. Meanwhile, IGBTs (Insulated Gate Bipolar Transistor) have largely eclipsed FREDFETs in frequency converters, motor controllers and similar because they are often the better option in terms of price and in technical terms. Despite this, FREDFETs continue to play an important role in specific switch-mode power supply topologies. One of these, which is becoming increasingly popular, is the ‘LLC converter’. Ideally, LLC converters operate in the resonant mode, which is also referred to as ‘soft switching’. For this topology, STMicroelectronics offers MOSFETs with characteristics that are precisely optimized for this purpose as well as LLC controllers (analog and digital), including integrated  600V half-bridge drivers.

 

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Image 1: LLC Bridge Circuit
Image 1: LLC Bridge Circuit