SiC-Based High-Density Charger Plie Power Module Design

Zhong Ye, Ph.D. Senior Member, IEEE, Qixiang Han, Hailong Yang

Inventchip Technology, China

Abstract

With more charger stations being built in urban

commercial area, the physical size of charger

piles becomes a main concern. Efficiency and

power density of charger modules directly impact

the charger stations investment capital and

operation cost. This paper proposes a new

AC/DC power conversion architecture, which

uses a resonant DC transformer fed by a three-

phase current-source PFC for battery charging.

By eliminating DC bulk capacitors at the PFC

output and employing SiC MOSFETs to simplify

the power topologies and operate at higher

switching frequency, both efficiency and density

are improved. A 20kW charger module prototype

was built to validate the concept and benchmark

existing design.

1. Introduction

Electric vehicles (EVs) are a rapidly growing

segment of the automotive industry owing to the

improved technology, low carbon footprints, and

government policy incentives. Currently, the

electric vehicle industry is undergoing a

technological transformation to improve vehicle

range with charging infrastructure. Electric vehicle

automakers and charge service enterprises are

investing heavily in charging station infrastructure

in order to support long-range battery electric

vehicles and improve EV drivers’ experience.

Government incentives and automakers’

initiatives for charging infrastructure development

are the key factors driving the growth of the global

electric vehicle charging stations market. The

market is projected to reach around UDS 30

billion by 2027 at a CAGR of 36% from 2019 [1].

The level 3 DC charging station sub-segment is

accounted for the largest share of the overall

electric vehicle charging stations market and Asia

pacific will command near 50% of the market

during the forecast period. The large share of this

segment is mainly attributed to rising demand for

setting up of charging infrastructure at convenient

urban commercial sites. Expensive commercial

real estate drives investors and developers to

squeeze charger pile sizes and increase charging

power. That leads to the increasing demand of

high density charger pile modules. To achieve

high density design, the power converters need to

operate at higher switching frequency with an

equal or better efficiency.

Currently, charger pile modules of the state of art

design and in volume production almost all use

650V Si MOSFETs in order to get a decent power

density and efficiency out with an acceptable

BOM cost. For a design with power over 6 kW, 3-

phase input becomes necessary. Since the

intermediate bus voltage exceeds 650V device

derating requirement, three-level topologies or

series-connected converters are the only choice

for the design. Fig. 1 shows three main topologies

used in 20kW-68kW commercial product designs.

Fig. 1a is the topology using a Vienna PFC and a

three-level phase-shifted full bridge DC/DC

converter[2]; Fig.1b topology consists of a Vienna

PFC and two series-connected three-phase LLC

converters[3,4,5]; these two are isolated solutions.

Fig. 1c shows a non-isolated solution with an I-

type Vienna PFC and two series-connected multi-

phase-interleaved buck converters. All of

topologies basically use two-stage approach,

which requires bulk capacitors to decouple the

PFC and DC/DC stages. The bulk capacitors

occupy around 20% of the module space, which

becomes an obstacle to density improvement.

Vienna PFC topologies have many

advantages[6,7,8], but it seems there is no other

good way to create an output voltage central point

without using series-connected bulk capacitors.

Complicity of the topologies is another issue, for

which a large PCB space can be consumed by

the gate driver circuitry. The non-isolated solution

is a good approach to improve charger module

density by eliminating high frequency power

transformer. This solution would require an on-

site AC power transformer to provide isolation

and safety protection.

With SiC MOSFETs getting more mature and

affordable, it opens up a new dimension for

engineers to use simple topologies and improve

overall design. 1200V SiC MOSFETs found a

sweet spot of applications where bus voltage is

over 650V and operating frequency needs to be

above 20kHz for hard switching and 50kHz for

soft switching in general. High power density

charger pile module would be a good playground

for SiC MOSFETs to exercise their merits.

(a)

(b)

(c)

Fig.1. Main topologies used in commercial designs.

Now, to reduce or eliminate the bulk capacitors

abovementioned, the question comes to what

side effect it may have to charge a battery with

AC line-frequency current ripple. [ 9,10,11]

research found that current ripple under 5kHz had

negligible impact on Li-ion battery performance

and lifetime, and interestingly the test shows the

batteries’ impedance reaches the lowest value at

around 100Hz. The authors took the advantage of

this battery characteristic and proposed a fast

pulse-charging method for Li-ion battery charging.

There are not many literatures found on this topic,

but it is reasonable to believe that hundreds’ Hz

current ripple at small percentage of charging

current amplitude is safe for Li-ion battery

charging.

Based on this assumption and by employing

1200V SiC MOSFETs, a new AC/DC power

architecture is proposed for charge pile module

design. The architecture uses an efficiency-

optimized resonant DC transformer (DC-X) to

isolate a three-phase current-source PFC’s output

current and to charge a battery pack directly. By

using simpler but more efficient topologies and

eliminating PFC output bulk capacitors, higher

power density of charger module design becomes

possible. For the detail discussion, this paper

organizes coming sections as follows,

Section 2: High Density Power Architecture

Section 3: Power Stage and Control Design

Section 4: Experiment and Benchmarking

And the final part is Conclusions.

2. High Density Power Architecture

Since charger stations need to support a wide

variety of electric vehicles charging, the charger

output voltage is required to range from 330V to

750V generally. To shorten charging time,

constant power control is preferred, which allows

the chargers to provide a higher charging current

when the battery SOC ( State Of Charge) is low.

However, constant power control adds a

significant design challenge to engineers in terms

of thermal design. With wide output voltage range

requirement, traditional phase-shifted full bridge

converters and LLC converters suffer from a

higher power loss due to wider duty cycle or

switching frequency regulation range. To mitigate

this issue, [12] uses dual bridges with phase-

shifted control to effectively achieve seamless

series and parallel connection change of two

rectifier outputs, but two phase current sharing is

a remained challenge. [13,14,15] proposes wide-

range input and/or constant-frequency LLC

converters. These are good topologies, but

adding two additional MOSFETs and

corresponding control circuit each phase in this

product design would cripple their advantages.

The concept of DC-X battery chargers with a

single-phase current source PFC has been

evaluated by previous research and good results

were achieved [16]. This paper extends the

research to a three-phase input, wider battery

voltage range and higher power level [17,18].

DC Output

3-Level Phase-Shifted DC/DC

Converter with Winding Switch

Vienna PFC Converter

Three-Phase

AC Input

Three-Phase

AC Input

DC Output

Vienna PFC Converter

Interleaved Buck Converter

Vienna PFC Converter

Series-Connected

3-Phase LLC DC/DC Converter

TA1

TA2

TA3

DC Output

Three-Phase

AC Input

TB1

TB2

TB3

TA1

TA2

TA3