The Hows and Whys of Digital Power Supply Isolation

With the rapid growth of communication infrastructure and the internet, techniques for digital control are becoming increasingly popular in the power systems of  networking, telecom, and computers due to their appealing benefits such as advanced control algorithms, flexibility, low sensitivity to external noises,, parameter variation, and component count reduction, system communication.

Digital power supplies are commonly used in high-end server, telecom brick modules, and storage among other applications. Isolation is frequently needed for these applications.

The challenge of isolating digital power supplies is to send digital or analog signals across the power isolation barrier while maintaining high accuracy, speed, and compact size. 

Why You Need Isolation

Compliance with safety standards is essential when designing a power supply to protect operators and other personnel from electric shock and hazardous energy. Isolation is a critical method for meeting safety requirements. Many organizations around the world, including VDE and IEC in Europe and UL in the United States, have specified different levels of input and output voltage—both steady state and transient state.

In UL60950, for example, five types of insulation are introduced:

  • Functional insulation: Insulation that is only required for the equipment to function properly.
  • Basic Insulation: Insulation that provides basic protection against electric shock.
  • Supplementary insulation is used in addition to basic insulation to reduce the risk of electric shock in the event that the basic insulation fails.
  • Insulation that consists of both basic and supplementary insulation is referred to as double insulation.
  • Reinforced Insulation: Under the conditions specified in this standard, reinforced insulation is a single insulation system that provides some protection against electric shock and is equivalent to double insulation.

Primary Side Control and Secondary Side Control Comparison

The power isolation control methods are classified into two types based on the position of the controller: primary side control and secondary side control. The table below compares the functions of the primary and secondary side controls. UVP and OVP stand for undervoltage and overvoltage protection, respectively.

Function Primary Side Control Secondary Side Control
Power Up Simple or direct dc regulated power supply is required to power the controller Auxiliary isolated power is required to power the controller
Gate Drive The  gate driver’s synchronous rectifier does not need isolation; the switch gate’s primary side needs isolation. The  gate driver’s synchronous rectifier does not need isolation; the switch gate’s primary side needs isolation.
Input UVP/OVP Isolation is not required. Isolation is required
Output UVP/OVP Isolation is required. Isolation is not required.
Control Loop Isolated control loop is required to regulate the output voltage. Isolated control loop is not required.
System Communication Isolation is required. Isolation is not required.
Remote on/off Isolation is not required. Isolation is required.

Secondary Side Control

The ADP1051 is an advanced digital power controller with a PMBus interface from Analog Devices that targets high power density and high-efficiency applications such as intermediate bus converters. 

The signals of ADP1051 PWM need to cross the isolation boundary. There are three approaches discussed: gate drive transformers, digital isolators, and isolated gate drivers.

Gate Drive Transformer

ADP1051, the secondary controller, sends PWM signals to the ADP3654, which is a dual-channel, MOSFET driver. After that, the ADP3654 powers a gate drive transformer. The motor drive  transformer, which transfers drive signals from the secondary side to the primary side, drives the primary side MOSFETs.

Digital Isolator

When transferring PWM signals from ADP1051 to the primary side half bridge driver, the ACPL-064L digital isolator is used for digital isolation.

The digital isolator solution is more compact, dependable, and simple to use compared to complicated gate drive transformer design. The duty cycle is not limited, and there is no saturation problem with this solution. This solution can achieve a high power density design because it saves more than 50% of PCB space.

Isolated Gate Driver

The ACFJ-3262, 10 A, isolated half bridge gate driver can provide independent and isolated high-side and low-side outputs to further simplify the design, integrate electrical isolation, and provide strong gate drive capability.

The isolated gate driver ACFJ-3262 is set up as a bootstrap gate driver to drive a half bridge. CBST is an external bootstrap capacitor and DBST is an external bootstrap diode. The bootstrap capacitor is charged by VDD via the bootstrap diode when the low side MOSFET Q2 is on during a cycle.

An ultrafast diode is required to minimize power dissipation, have a low forward voltage drop, and a fast reverse recovery time.

Primary Side Control

Primary side control is more popular in some low-cost applications because it does not require an auxiliary isolated power supply and has a simple control architecture.

There are three approaches discussed in terms of the isolating control path: linear optocoupler, general optocoupler with standard amplifier, and isolated amplifier.

Linear Optocoupler

In general, isolating the output voltage in a digital power supply necessitates fast and accurate isolating feedback. Optocouplers are frequently used to send analog signals from the secondary side to the primary side.

With a general optocoupler used directly in the feedback loop to transfer the output voltage, it is extremely difficult to guarantee the output voltage accuracy. To transfer the compensation signal rather than the output voltage, a general optocoupler is combined with an error amplifier.

Because the ADP1051 already has digital loop compensation built in, the compensation signal is no longer required. One solution is to use a linear optocoupler to transfer the output voltage.

General Optocoupler with Standard Amplifier

A general optocoupler and a standard amplifier can be used in another circuit to achieve primary side control. High-output voltage accuracy is acquired while avoiding the optocoupler’s wide CTR range caused by temperature changes.

The measurement results show that the output voltage variation is in the 1% range, with a CTR range of 100-200%.

CTR is calculated using the following formula:

When CTR varies with temperature, the amplifier’s output compensates to maintain high output voltage accuracy. It should be noted that the amplifier’s stable operation point and swing range should be well designed to satisfy the CTR variation with temperature requirement in the event that the amplifier’s output saturates.



Post Author: Wyatt Canton'