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Efficiency, Isolated Bidirectional DC-DC Converter Reference Design for UPS

1 Description

The TIDA-00951 design provides a reference solution for a 2-kW isolated bidirectional DC-DC converter capable of power transfer between a 400-V DC bus along with a 12- to 14-cell Lithium battery pack to be used in UPS, battery backup and power storage applications.

This TI Design works as a >93% efficient, current fed, active clamped boost converter with ZVS within the backup mode and voltage fed full-bridge batter charger with >93% efficiency in the charging mode. This TI Design has built-in protection for DC bus overcurrent and overvoltage and battery overcurrent.

2 Features

  • Digitally Controlled Isolated Bidirectional DC-DC Converter
  • Operates as Active Clamped Full Bridge Boost Converter With ZVS For those Low-Voltage Switches at High Loads
  • Operates as Active Clamped Voltage Fed Buck Converter With Synchronous Rectification to Improve Efficiency When Charging Battery
  • Wide Operating Range From 36- to 60-V Battery and 300- to 400-V DC Bus
  • Cost Optimized Design Using 100-V FET on LowVoltage Side, Eliminates Requirement of Paralleling Multiple FETs up to 2 kW
  • Built-in Cold Start Procedure and Fast Mode Transfer (< 100 uS) From Battery Charger to Backup Power Supply
  • Onboard Isolated Communication Interface for CAN, I2 C, and RS-485

3 Applications

  • Server PSUs and Telecom Rectifiers
  • Uninterruptible Power Supplies ( UPS )
  • Industrial Power Supplies
  • Battery Chargers
  • Energy Storage Systems

4 System Description


Most backup power equipment for example DC inverters, home inverters, industrial DC-UPS, and storage banks require an exchange of power in the battery towards the load and the other way around. Typical power system distribution architecture with battery backup is shown in Figure 1:

Figure 1. Top-level Architecture of Typical UPS System

During normal operation, the primary DC bus is regulated between 300 and 400 V through the grid supply of a building, factory, or house. Alternatively, the DC bus could be powered via a alternative energy source such as solar power generation or residential wind power generation, that is conditioned via a power conditioner to feed the DC bus. Battery acts as a power storage space, also it can be charged either through the grid or perhaps an external alternative energy source.

Conventionally, charging a battery via a DC bus and discharging the battery during power blackouts are implemented with two unidirectional converters, each processing the ability one way. Having a growing focus on compact and efficient power systems, there's increasing curiosity about using bidirectional converters, particularly in DC inverters, home inverters, and energy storage banks. A bidirectional DC-DC converter, capable of bilateral power flow, provides the functionality of two unidirectional converters in one converter unit.

The TIDA-00951 design is definitely an isolated bidirectional DC-DC converter designed to exchange the ability from a 300- to 400-V DC Bus and 48-V battery banks. The look includes a full-bridge power stage around the high-voltage (HV) side, which is isolated from the current-fed full-bridge stage on the low-voltage (LV) side. Throughout the presence of the DC bus (normal conditions), the design operates in buck mode and expenses battery with constant current before the battery voltage is within regulated limits. During blackouts, the design operates because the current-fed full-bridge converter to boost the power from the 48-V battery (36- to 60-V input) towards the 380-V DC bus and supports the load with backup.

System Description

The transition or change with time in the charge to backup mode is very crucial for ensuring continuity of capacity to the loads. The TIDA-00951 has transition duration of less than 100 us, which reduces the quantity of bulk capacitance required for the machine to provide power throughout the transition time.

This TI Design operates at peak efficiency of 93% in buck mode (as charger) and 94% in boost mode (during discharge). The high discharge efficiency provides a high run time in the battery. Operating at a high switching frequency of 100 kHz, the look has a compact form factor of 185 mm × 170 mm for the power level of two kW.

The TIDA-00951 design is optimized for component count, cost, and performance. Various parameters of the design like regulation, efficiency, output ripple, transition time, startup, and switching stress across the devices were tested and documented in the following sections.


4.1 Key System Specifications

Table 1. Key System Specifications

Input battery voltage (VBAT) 36 44.4 60 V
Input battery current (IBAT_MAX) 60 A
Output bus voltage (VBUS) 300 380.0 400 V
Output bus current (IBUS_ MAX) VBAT > 40 V 5 A
Line regulation 1 %
Load regulation 10% to 100% load 1 %
Output voltage ripple
Input voltage ripple
Average efficiency 20% to 100% %
Full load efficiency %
Input bus voltage (VBUS) 300 380.0 400 V
Input bus current (IBUS_ MAX) 60 A
Output battery voltage (VBAT) 36 44.4 60 V
Output battery current (IBAT) 16 A
Operating ambient –10 25 55 °C
Board size Length × Width × Height 185 × 173 × 8 mm


5 System Overview

5.1 Block Diagram

Figure 2 shows the high-level block diagram from the TIDA-00951. The DC-DC converter is made of a current-fed full-bridge converter on the battery side and a full-bridge on the 380-V bus side. The charge of the machine is through the C2000 present on the TIDA-01281 control card. The TIDA-01159 isolated gate driver card can be used they are driving the full bridge around the 380-V bus side. High-side inductor current sensing is performed using the TIDA-01141 board.

5.2 System Design Theory

The isolated bidirectional DC-DC converter has two major modes of operation. When it's being employed as a backup power supply, it operates as an active clamped current-fed boost converter transferring power from the battery towards the 380-V DC bus. When operating as a battery charger, the DC-DC converter functions as a buck converter transferring power from the 380-V DC bus to the battery.

Apart from the two major modes, it comes with an additional way of the cold starting the system. This mode is used to begin in the TIDA-00951 should the HV DC bus is totally discharged before board start-up.

The working of the isolated bidirectional DC-DC converter design is detailed in the following sections.

5.2.1 Boost Mode Topology Description

When working in boost mode, the system must boost an input voltage between 36 to 60 V to a 380- V DC output. There are multiple topologies that can be considered for this TI Design.

Broadly speaking, the potential topologies can be classified into voltage- or current-fed topologies come with an input inductor, that is connected to the power stage. However, a voltage-fed converter connects the input filter capacitor towards the power stage. The presence of this input inductor gives the following advantages:

  • Boosted voltage reduces the stress in the transformer and utilization
  • Avoids flux imbalance issues in the power stage
  • Lower stress on the input filter capacitors because of reduction in the current ripple due to the input inductor

The TIDA-00951 works as an active clamped current-fed full-bridge converter in boost mode. However, there are several advantages in using a current-fed converter, one primary disadvantage is the huge spike within the current-fed converter at MOSFET turnoff. This turnoff requires some form of snubbing using either an energetic or passive snubber.

In the TIDA-00951 design, an active clamp compromising of the capacitor and a MOSFET has been used to apply an active snubber. The benefit of this active clamp isn't that only does it recover the leakage energy, but it also helps in achieving ZVS for that primary LV MOSFET at turnon, thereby lowering the switching losses.

When working as a battery charger, the TIDA-00951 functions as a voltage-fed buck converter. It transfers power from the HV DC bus and charges battery in constant-current/constant-voltage (CC/CV) mode with a current limit of 16 A.

The following two sections let you know that the converter in the backup supply mode works. Boost Mode Working: Active Clamp

power stage from the TIDA-00951 is shown in Figure 3.

The switches Q1 to Q4 are the LV-side full-bridge MOSFET. The switches Q6 to Q9 form the HV-side fullbridge MOSFET. The capacitor Cclamp and switch Qclamp make up the active clamp.

When the system functions as a current-fed full bridge, transferring power in the battery to the DC bus, the active clamp stores the additional leakage energy when the MOSFET Q1 to Q4 turnoff, thereby limiting the turnoff spike around the MOSFET. Additionally, by manipulating the switching from the MOSFET Q5, the main LV MOSFET could be switched on in or near to 0 V, thereby reducing the turnon switching losses.

The modality of implementing this scheme is shown in Figure 4.

Figure 4. Active Clamp Waveforms

Take the switch pair Q1 and Q3 for example to explain the significant from the active clamp. When the switch pair Q1 to Q3 switch off, the present through FETs before turnoff get transferred and begin flowing in to the clamp capacitor and the body diode of Qclamp. Since the clamp current is flowing with the body diode of Qclamp, it may be turned on following a short time (Tdelay_1) in ZVS condition as shown in Figure 4.

Now, before the switch pair Q1 and Q3 is switched on again, the clamp FET Qclamp is switched off. Since the direction of the Iclamp has reversed and it is now flowing through the channel of the Qclamp, Iclamp instantly comes to zero.

Because the current with the leakage inductor cannot change instantly, part of the current flowing with the FET Qclamp, starts to flow with the body diode of FET Q1 and Q3. This starts to discharge the COSS of FETs Q1 and Q3 and causes ZVS to occur. After this, FETs Q1 and Q3 could be switched on under ZVS or close to ZVS condition, thereby lowering the turnon loss. The delay in the point of turnoff of the clamp FET Qclamp and turnon of Q1 and Q3 is marked as Tdelay_2 in Figure 4. Boost Mode Working: Cold Start

Figure 5 shows the LV-side full-bridge converter along with the start-up clamp circuit.

Figure 5. Cold Start Clamp Circuit

During the system start-up, if the HV DC bus is completely discharged. The TIDA-00951 starts up utilizing an additional flyback winding present around the inductor L1. In this mode, the LV-side full bridge doesn't work like a current-fed converter but because a flyback converter. It works within this mode until the HV bus reaches 270-V DC and then the system switches to being employed as a current-fed converter.

In Figure 5, the MOSFETs Q1, Q2, Q3, and Q4 form the LV-side full bridge. Qclamp and Cclamp form the active clamp. The flyback winding on the inductor L1 can be used to temporarily charge a little capacitor, that is then boosted and fed to the HV DC bus output capacitors while using boost converter formed by Q10, D10, and L3.

Figure 6 shows the PWM waveforms from the LV MOSFETs Q1 to Q4, clamp MOSFET QClamp, and also the boost MOSFET Q10. The figure also shows the current-fed inductor current waveform and the drain-to-source voltage waveform of the low-side bridge MOSFET.

All the MOSFETs around the LV bridge are turned on simultaneously. This charges the current-fed inductor. When these MOSFETs are switched off, the present in the current-fed inductor is transferred to the flyback winding and is kept in the capacitor Ccold_start.

The boosted MOSFET Q10 will be switched on to charge the boost inductor L3. This energy is then transferred to the HV bus output capacitors. Low-Side Current Sensing Circuit

In the TIDA-00951, low-side current sensing is implemented to measure the battery current on the LV side while using OPA376.

Because the battery current is bidirectional in nature, the creation of the OPA376 is 1.2 V by using the LM4041A12 shunt voltage reference. This is shown in Figure 7.

Low Side Current Sensing SGND

The OPA376 difference amplifier measures the present across a 0.25-mΩ current sense resistor formed by the parallel mixture of resistors of R20 and R29. Isolated Voltage Sensing

The control electronics (TIDA-01281) on the TIDA-00951 board is referred to the LV battery-side ground. In order to sense the voltage over the isolated 400-V bus, an isolation amplifier circuit based around the AMC1301 isolated amplifier and OPA376 op amp can be used.

The differential creation of the AMC1301 is scaled and converted to a single-ended output for connecting directly to the MCU. Figure 8 shows this circuit.

Courtesy: www.ti.com