SPST Bipolar Power Switches Simplify Power Supply Testing

Abstract: This application note describes an isolated SPST (single pole/single throw) bipolar power switch for generating 200A, 75V transient signals. The switch can be used to test power supplies or power ICs and to test high-speed circuits. The switch on and off times can reach nanoseconds.

introduction

Handle the necessary high currents and pulse voltages. Such a switch must be able to connect circuits of various topologies, depending on the type of power supply being tested and its application.

In some cases, commercial test loads can be used or a set of test procedures can be developed. If one side of the switch is connected to the common terminal of the power supply, the test process will be relatively simple. Otherwise, the switch driver must be designed, which complicates the design. A switch that is flexible enough to support power supply transient fault testing will be a very useful test tool. In summary, the requirements for such a switch can be summarized, including maximum rated voltage and rated current, which are used to test most medium-power power supplies on the market today.

Such a switch should be able to handle a current of 100A and withstand an open-circuit voltage of at least 75V. The switch needs to meet current and voltage requirements in both directions, because some tests will produce current ringing and some power supply circuits require bipolar outputs. The switch's on and off speed should be kept within tens of nanoseconds to observe the transient response of the circuit. The series resistance of the switch must be low enough, and the series inductance must also be very low, which means that a compact physical structure and a very short current path are required. In addition, the switch is required to provide electrical isolation and have very low output capacitance to ground. In summary, the basic requirement for the switch is that it does not affect the performance and response of the circuit when the switch is connected to the circuit.

Circuit Description

The switch design shown in Figure 1 meets many of the above requirements and is implemented using a digital isolation coupler with an end-to-end capacitance of less than 1pf. The total propagation delay is 80ns and the output rise time is close to 40ns. The output stage consists of two low RDSON MOSFETs that can handle bipolar 200A, 75V power supply transients. The switch

elements (two output MOSFETs connected inversely) have a 7mΩ on-resistance and 25nH on-inductance. In the on state, they behave as a linear resistor for bidirectional current (including zero), similar to a wire connection, without introducing harmonic distortion.


Figure 1. This circuit uses a 5V logic signal to control independent (isolated) power switches Q1–Q2 and can handle 200A, 75V pulse signals.

For small resistance loads that draw currents greater than 50A, the switch rise time (defined as the instantaneous turn-on) is determined by the on-inductance. At lower currents, the rise time is less than 40ns, and the fall time (instantaneous turn-off) is mainly determined by the load impedance.

The power supply for the isolated side of the circuit (the switch) is a set of three 3V lithium-ion primary batteries in series (CR2025 lithium manganese batteries). For a switching frequency of several kHz, this battery with a nominal value of 170mAh can be used continuously for more than a month. For a common test platform, the battery life is about 3 months (the left side is always connected).

The input is a digital signal of 0V to 5V, requiring only rising and falling edge times less than 20ns and a minimum pulse width of 50ns (on or off). When the conduction current is less than 18A, the switch is in an indeterminate on or off state.

In Figure 1, IC1 and IC2 form an edge detection circuit, applying a narrow pulse to one end of the primary of T1, with polarity depending on the input edge, and the other side is kept low. The secondary side of T1 is connected to a non-inverting logic buffer (input to output), which is formed by one channel of the dual-channel low-side power MOSFET driver of IC3. This buffer acts as a bistable circuit (i.e., a flip-flop), which is set when a positive pulse is applied to the primary of T1 and reset when a negative pulse is applied. The output of the bistable circuit is a copy of the circuit input (the digital input acting on the edge detection circuit).

The other half of IC3 is connected in parallel with the two drivers of IC4, whose inputs are connected to the bistable outputs. The parallel output drives the gates of two low-RDSON MOSFETs (IRFB3077) connected in reverse. The drains of the two MOSFETs are connected to the external power supply, and the two gates and two sources are connected together. The three drivers in parallel can effectively increase the switching speed of the power MOSFETs. Because IC2–IC3 share the current, each driver can provide a peak current of 4A, and the total current can reach 12A after parallel connection. The source of the MOSFET is connected to the negative terminal of the battery.

The input logic of the MAX5048 simplifies the design of the edge detection circuit, and the low quiescent current of the MAX5054 helps to extend the battery life. Therefore, similar but different ICs are used in this design for low-side (control and isolation, IC1, IC2) and high-side (power driver, IC3, IC4) drivers.

Figure 2 shows the equivalent circuit of the power switch, including the main parasitic components. For the entire power supply circuit, the continuous power that the switch can withstand depends on the heat sink. Heat sinks significantly increase the parasitic output capacitance and are not included in this design. Some additional conditions are required when handling 200A pulse currents. The pulse width must be limited to less than 8ms and the switching duty cycle must be limited to less than 0.5%. For the 80A transient signal, the pulse time is not limited and the duration is longer (up to 50ms), but the duty cycle must not exceed 3%.


Figure 2. This power switch circuit is an equivalent architecture of the circuit in Figure 1, including the main parasitic elements.

When switching an unclamped inductor at room temperature, the energy that the circuit can absorb is 280mJ (single pulse, non-repeating) or 200mJ (pulse with a maximum duty cycle of 1%).

The coupling transformer design requires small size and low winding capacitance: one turn on the primary side and two turns on the secondary side, wound on a Fair-Rite 7.5 x 7.5m ferrite bead. The transformer construction determines the maximum voltage difference between the switch load and the switch control circuit. When using ordinary enameled wire insulation structure, 1kV isolation can be provided. If Teflon or better insulation materials are used, more than 1kV isolation can be provided. For designs requiring higher isolation voltage, the packaging must also be considered. The ferrite core of

T1 is a conductor and cannot be connected to both sides of the switch at the same time. There is no latch protection inside the switch, and the state of the lithium battery must be verified before operation. There is no circuit to ensure that the switch is in a certain state (on or off) after power is applied. Therefore, the switching power supply must be turned on before connecting other power supplies. The switch state is determined by the transient that first acts on the input terminal, and the switch is turned on and off at least once while powering other parts.

Test Circuit

The top waveform in Figures 3–5 is the digital input, and the bottom waveform is the 5µs pulse waveform observed through a 0.25Ω resistor load, which is connected to a 50V power supply through a switch. Because the voltage waveform acts on a low-inductance thin film resistor, it can be approximated to represent the current waveform of the switch. The approximately 200A pulse waveform in Figure 3 shows the overshoot and rise time (60ns to 80ns), and the rise time is affected by the parasitic inductance and capacitance of the high-side current path. Figure 4 shows the rise time and turn-on delay of this pulse; Figure 5 shows the fall time and the transmission delay when turning off. Figures 6–8 show the same waveforms with a 5Ω load, a 10A pulse, and a power supply voltage of 50V. The rise delay is close to the inherent rise delay of the MOSFET of 30ns to 40ns, which is limited by the package and source impedance.


Figure 3. Test results of Figure 1, (1) control signal, (2) 5µs pulse measured across a 0.25Ω resistor, with a power supply voltage of 50V.


Figure 4. Rise time and turn-on transmission delay based on Figure 2, with a scan rate of 40ns/cm.


Figure 5. Fall time and turn-off propagation delay based on Figure 2, with a scan rate of 40ns/cm.

Figure 6. Test results for Figure 1, (1) control signal, (2) 5µs pulse across a 5Ω resistor, with a 50V supply voltage.


Figure 7. Rise time and turn-on propagation delay based on Figure 6, with a scan rate of 40ns/cm.


Figure 8. Fall time and turn-off propagation delay based on Figure 6, with a scan rate of 40ns/cm.