DC-DC level LLC resonant converter topology for OBC

Choosing a DC-DC converter solution for an on-board charger (OBC) is based on efficiency, performance, and power density goals, so SX resonant converters. This article introduces the popular LLC and LLC-derived bidirectional converter topologies described in the literature.

Choosing a DC-DC converter solution for an on-board charger (OBC) is based on efficiency, performance, and power density goals, so SX resonant converters. This article introduces the popular LLC and LLC-derived bidirectional converter topologies described in the literature.

introduce

As shown in Figure 1.1, a typical OBC architecture has a bidirectional front-end ac-dc stage followed by an isolated bidirectional dc-dc converter to charge the high-voltage battery. Designers must meet performance, efficiency, and power density goals across the entire grid and battery voltage range. For the ac-dc level, totem pole PFC is the SX solution. The charging algorithm is implemented in the DC-DC stage. dc-dc switches at high frequencies and requires a soft-switching topology in both directions, even if wide bandgap devices are used.

DC-DC level LLC resonant converter topology for OBC

Typical OBC power system

Figure 1.1: Typical OBC power system

Phase shift full bridge [1] It is a suitable topology, but there are problems such as limited zero voltage switching (ZVS) range, duty cycle to obtain ZVS, and buffers for secondary devices. The dual active bridge also runs with ZVS, but has the performance of ZJ performance fixed output. For high power, resonant converters are SX because they provide soft switching in all devices and have low EMI even at high frequencies.

The small number of components, the use of transformer leakage inductance for resonance, and the absence of snubber/clamp circuits are other additional advantages. The FET-based rectifier makes the converter bidirectional. This article introduces LLC and LLC derivative topologies for DC-DC, and introduces the OBC design challenges of these converters.

Resonant DC-DC converter for Bi-OBC

The DC-DC level specifications of a typical 6.6kW OBC are shown in Table 2.1. The design is for ZG power, current and thermal stress are determined for the charging mode. Please note that the efficiency requirements for both modes are very strict.

Range value review
Battery charging module
Input voltage 400V in two-wire
40Vpk-pk ripple at frequency
Nominal output voltage 330V
Output voltage range 200V-450V
Output power 6.6kW
Output current ZD value 20A330V and below
Discharge method
Nominal input voltage 330V
Input voltage range 200V C 450V
Output voltage 400V input to grid-connected Inverter stage
ZD output power 3.3kW
General specifications
Efficiency target>98%, overall efficiency is as high as 96%
Isolated 3 kV
LLC resonant converter

The LLC power level is shown in Figure 2.1. This circuit has two full bridge circuits separated by an isolation transformer. The transformer ratio is set for the nominal operating voltage. The gain of the resonant tank is a function of the resonant components (Lm, Lr, and Cr), load, and switching frequency.

LLC converter power stage

DC-DC level LLC resonant converter topology for OBC

Figure 2.1: LLC converter power stage

The LLC converter design procedure is not straightforward, and it takes some iterations to determine the ZJ resonant tank component value.The design steps are summarized as follows

1. Set the transformer turns ratio (N) according to the nominal working input and output voltage (400V input and 330V output)

2. Determine the ZD and Z small gain requirements according to the converter parameters in Table 2.1. The ZD gain is evaluated by the ZD output voltage and the Z small input (the Z small voltage considering the line frequency ripple content in the PFC output).Similarly, the peak value of the input voltage will be used for Z small gain calculation

3. Calculate the switching frequency range. This will be an iterative process, requiring adjustment of the fuel tank parameters Q (quality factor) and M (the ratio of Lm to Lr)

Set the resonance frequency value. High frequency is preferred to reduce the size of the transformer. In addition, the output filter capacitor and resonant capacitor values ​​decrease with frequency. However, the turn-off loss of the transformer and FET must be monitored when determining the frequency.

Determine the ZD Lm value at resonance, which needs to discharge the Coss of the FET and help the ZVS of the primary device to turn on

Set a value for M as a start. A high value of M indicates high magnetizing inductance and low circulating energy, but the achievable gain is limited. For lower M values, high gains can be achieved in a narrow frequency range. The resulting magnetizing inductance is relatively small, and the associated circulating current and losses are relatively high. A value between 6 and 10 is sufficient to change from [6] Start.

Select Q according to the ZD gain requirement at full load. If the gain is not enough, the value of M must be reduced. The gain range should be realized over the entire load range or Q range.

The frequency range of the relative gain should be small, and the small Z frequency should have little effect on the size and loss of the magnet.Reiterate the design of Q and M to meet gain and frequency range standards

4. According to the values ​​of M and Q, Z finally determines the values ​​of Lr, Cr and Lm.

The LLC converter has bidirectional power flow capability. But in the discharge mode, the magnetizing inductance appears directly at both ends of the battery, followed by Lr and Cr, resulting in a series resonant converter type configuration.[4]The gain curve of the medium LLC in the charge and discharge curve is shown in Figure 2.2. The discharge curve shows that the converter has no voltage gain, which will cause the output to be unstable.exist [2] In the discharge mode, the LLC switches at the resonant frequency, and a boost converter stage is added after the LLC adjusts the input of the PFC stage. In battery charging mode, the boost stage is bypassed by the relay. However, this method increases component cost and system size.

LLC charge-discharge mode gain curve

Figure 2.2: LLC charge-discharge mode gain curve

CLLLC resonant converter

The bidirectional CLLLC resonant converter with 5 resonant components is shown in Figure 2.3. The resonant tank is symmetrical, and the converter has approximately similar gain curves in charging and discharging modes.

CLLLC converter power stage

Figure 2.3: CLLLC converter power stage

The design method of CLLLC power stage is similar to LLC converter. The secondary resonant components are all called primary, and the equivalent circuit produces a transfer function. In order to simplify the design steps, assume that the reflection Lrs and Lrp are the same, and set the ratio of reflection Crs to Crp. Adjust the equivalent M and Q values ​​to meet the gain and frequency range standards in the two modes. When determining the value of M, ensure that the gain curve is monotonously decreasing, without multiple peaks, so as to achieve linear control within the entire operating frequency range.

The CLLC resonant converter is derived from CLLLC, which eliminates the secondary side resonant inductance. However, Crs needs to be used to adjust the gain curve of the discharge mode. If the transformer leakage inductance is also to be used, the equivalent configuration becomes the CLLLC type.[3] The CLLLC design examples and experimental results of 3.5kW OBC are introduced in.

CLLLC with variable DC link voltage

The frequency change of the output regulation causes the converter to deviate from resonance, the point where the converter is optimized. In order to keep the frequency swing at the low Z limit, the DC bus voltage will vary according to the required output voltage. Adjust the transformer ratio to make the Z-small output voltage correspond to the 400V DC bus, and then the DC link changes linearly according to the set output reference.[4] The gain curve presented by the design in Figure 2.4 is shown, and the frequency range is significantly reduced.

Gain curve of fixed DC bus and variable DC bus CLLLC

Figure 2.4: Gain curve of fixed DC bus and variable DC bus CLLLC

in conclusion

The resonant converter is undoubtedly the SX of OBC dc-dc conversion. With modern wide band gap devices, designers can easily achieve high efficiency at high frequencies. The article describes popular resonant converter configurations based on LLC converters. The design method suitable for bidirectional OBC specification in the literature is introduced.

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