How to use SCR to easily drive AC/DC converter startup?

I. Introduction

The market demand for new servers has grown rapidly over the past decade, with a CAGR of 11% from 2015 to 2022. The growth is being driven by the acceleration of paperless personal documents and digitalization of corporate offices; secondly , the integration of new media platforms into personal lives during the global health crisis has led to a significant increase in screen time as people work from home; and finally , the market will continue to grow rapidly with the rise and popularity of artificial intelligence. Against this backdrop, designing switching power supplies for servers is not easy, mainly dealing with the high heat dissipation issues and reducing the maintenance costs of such large, scalable devices, which are two major challenges facing power supply developers.

A solution based on thyristors to solve these two major problems came into being, and a solution was proposed to use thyristors to replace the traditional electromechanical switch design in the starting function of the AC/DC part of the switching power supply.

II. Advanced Technology

a) Principle

During the startup of the AC-DC power converter, the DC charging of the large-capacity capacitor will generate a large current 10 times higher than the nominal steady-state current of the system. This inrush current will cause a voltage drop on the AC power supply, thus affecting the normal operation of other devices connected to the same power supply. The IEC61000-3-3 standard defines voltage fluctuations and flickers.

This inrush current must be suppressed to protect the safety of electrical equipment and the reliability of power converters. In fact, the inrush current may trigger or burn out devices connected in series with the power supply, such as circuit breakers, fuses, capacitors or bridge rectifiers.

There are three solutions to suppress the inrush current that occurs when electrical equipment is connected to the power supply:

• Use a relay to connect an NTC or PTC thermistor in parallel, and short-circuit the starting resistor after the power returns to a steady state to transmit power and reduce the power loss of the resistor;

• Use SCR thyristors to replace the relays in solution 1;

• Designing soft-start configuration using SCR

See Figure 1 for a diagram of how each inrush current suppression solution works during the startup and steady-state phases.

Figure 1: AC/DC converter: inrush current suppression circuit topology

b) Find a suitable method for soft start

The previous standard topology was to use an electromechanical relay to create a bypass around the NTC thermistor that manages the inrush current. The same effect can be achieved by using an SCR to create a bypass. Now a more optimized method is to use a soft start topology.

By controlling the SCR switching operation with a phase angle, the voltage of the PFC output capacitor can be smoothly increased to the peak voltage of the AC line. The MCU controls the pre-charge current peak and synchronizes the phase angle step size of the SCR gate drive signal (Δt in Figure 2).

Figure 2: Soft start using pure SCR topology

It is not difficult to find that the ILINE peak value and the Δt value are related: the larger the Δt value, the higher the ILINE peak value and the faster the system starts.

c) Advantages of pure SCR inrush suppression topology

The soft-start topology allows designers to handle the inrush current that occurs during the startup phase of an application without electromechanical components and passive components (i.e., NTC or PTC), thereby reducing the bill of materials cost of the AC/DC rectification section.

By controlling the conduction of the SCR with an MCU, designers can easily set the line current and improve the startup time while meeting the IEC61000-3-3 standard.

SCR thyristors X1 and X2 are replacing the standard rectifier diodes on the lower bridge arm. The thyristor drive circuit is composed of a bidirectional thyristor Q1 and two small diodes D1 and D2.

Because of this driver configuration, the MCU can directly control the SCR conduction without the need for additional isolation circuitry and AC line polarity detection. As long as a positive bias voltage is applied, the SCR can conduct forward, so there is no risk of potential power loss when the gate current is reversed to the unused SCR.

This all-solid-state inrush current management solution has no bulky moving mechanical components, thus improving the reliability and life of the power supply. In addition, the application no longer has EMI noise caused by relay contact bounce. Compared with relays, thyristors have no aging issues.

Figure 3 compares the application performance of a 16A SCR and a 16A mechanical relay in terms of energy efficiency, power density, service life, acoustic noise, and electromagnetic interference.

Figure 3: Performance comparison between SCR and mechanical relay

III. Power loss in data centers is very serious

The main challenge facing data center SMPS designers is power loss. Even with the continuous improvement of cooling methods, whether it is water cooling or oil cooling, the primary means is still to limit the power loss of the converter's discrete power devices to make the switching power supply as energy-efficient as possible.

a) Energy consumption comparison between a 1500W power supply based on relays and a 1500W power supply based on SCRs

We measured the efficiency of the pure SCR solution on a 1500 W power supply unit (PSU). The original configuration of the power supply used mechanical relays, which we replaced with an evaluation board (STEVAL SCR002V1) developed by STMicroelectronics to implement a pure SCR startup topology on the power supply. Figure 4 shows the efficiency of the two solutions from 10% to 100% output load.

The energy efficiency of the SCR topology is exactly the same as that of the relay.

Figure 4: 1500 W power supply efficiency vs. load ratio

b) SCR loss optimization at 150°C junction temperature

Selecting the right SCR is very important to prevent high power losses and possible overheating. STMicroelectronics has developed a range of SCRs specifically designed for AC/DC converters.

The 150°C maximum junction temperature is a key parameter. Figure 5 depicts the on-state characteristics of a 16A SCR (TN1605H-8I) at high temperature. At 150°C junction temperature and RMS 6.5 A current (at 1500 W / 230 V power supply), the on-state voltage drop of the SCR is lower than the on-state voltage drop at normal temperature 25°C, so the power loss of the SCR will be lower when working at high temperature.

When operating at high temperatures, SCRs have other advantages: lower heat dissipation requirements, wider temperature margins, and higher reliability.

Figure 5: 16A SCR characteristics at high temperature on-state

VI. Circuit Implementation and Working Mode

The first question a designer has is “How to design the SCR gate circuit in a rectifier bridge?”

This question is easy to answer because the underlying gate circuit is made up of discrete components, directly controlled by the MCU, and no additional isolation devices are required for the interface.

To explain the working principle of the circuit simply, we only discuss the positive half of the AC line sine wave, and here is how the circuit operates: SCR X2 is turned on by Q1 through diode D2. Therefore, once the MCU activates Q1, X2 will also turn on. The gate current of Q1 comes from the MCU, while the gate current of X2 is the current received from the AC power supply through Q1 and D2.

Figure 6: Circuit operation

During the positive half cycle of the sine wave, the D1 diode is reverse biased so that no gate current flows through the gate of the X1 SCR, thereby preventing the increase in reverse losses due to SCR leakage current.

During the surge phase, the MCU controls the phase angle of the Q1 triac, so the X2 SCR is also controlled with a phase angle. The surge current flows through D3, the PFC output capacitor (C), the X2 SCR, and then back to the neutral line. Vdc is charged smoothly.

In the stable state, the PFC is turned on, the MCU controls the Q1 triac to be turned on in the full band, and the X2 thyristor is turned on. The same operation is performed in the negative half cycle of the AC power supply, using the same MCU I/O signal.

in conclusion

The pure SCR topology and its dedicated non-insulated driver can easily replace electromechanical relays and/or passive components to solve the inrush current problem when electrical equipment starts. This complete solid-state solution based on high junction temperature SCR is very suitable for applications with very high power density, such as SMPS power supplies in data centers. This solution has the following benefits: high energy efficiency; removal of mechanical parts, high reliability; and realization of simple non-isolated control circuits.

Author of this article:

Romain PICHON – STMicroelectronics Thyristor Applications

Benoit RENARD – Thyristor Marketing, STMicroelectronics