Design and application of high voltage negative power supply

Design and application of high voltage negative power supply

SLIC is an analog telephone interface with functions such as off-hook/on-hook signal transmission, ring tone generation, and ring current detection. It requires a -24V voltage when transmitting off-hook signals, and only a -72V voltage when generating ring tone signals. For IP phones or routers, SLIC generally requires a -48V voltage, while xDSL line drivers require -5V and -15V voltages, and wireless public telephones may require -24V and -48V voltages. All of these application power supply designs can use the MAC1856 synchronous PWM controller to obtain a low-cost design solution.

2 Working Principle and Characteristics of MAX1856

MAX1856 contains a low-dropout linear regulator (LDO). All internal circuits are powered by the built-in regulator. The maximum input voltage can reach 28V. The minimum voltage difference of the internal regulator is 200mV. As long as the LDO output voltage is higher than 2.7V, the internal circuit can work. Therefore, the minimum input voltage of MAX1856 is 3V, and the maximum output current of LDO is 12mA. If the gate drive current of the external FET is small, LDO should also be used to power other circuits. MAX1856 contains a current mode PWM controller, so it is ideal to use the flyback control structure to generate a negative high-voltage power supply. Its internal multi-input comparator can simultaneously process the output error signal, current detection signal, and slope compensation signal. In PWM mode, MAX1856 uses a fixed switching frequency. When the load is light, MAX1856 enters idle mode to provide higher efficiency.

The MAX1856 has an internal soft-start feature that prevents overcurrent and allows the use of smaller input capacitors. The operating frequency of the MAX1856 can be set between 100kHz and 500kHz, and for low-noise applications, an external clock synchronization mode can also be used.

3 SLIC power supply design

Figure 1 is a power supply circuit designed for SLIC. Its input voltage is 12V. One output is -24V/400mA and the other is -72V/100mA. The two output voltages can be adjusted by changing the external voltage divider resistor.

3.1 Setting the operating frequency and output voltage

The operating frequency of MAX1856 is determined by the resistor ROSC between the pin FREQ and the ground. ROSC is generally 50MΩ/fosc. If the operating frequency of 250kHz is selected, ROSC=200kΩ. The higher the operating frequency, the smaller ROSC can be, so that the peak current and resistance loss will be less, but at the same time, the switching loss and core loss will increase, and the gate drive current will increase.

The voltage divider resistor between the FB pin at the output and the REF pin determines the output voltage. Vout=VREF R1/R3. For the case where the dual output voltages are adjusted separately, since the feedback voltage threshold is 0, the total current flowing into the FB pin is ITOTAL=IR1+IR2=VREF/R3. Because the feedback resistor is connected to REF, ITOTAL must be less than 400μA. Selecting R3 can make ITOTAL between 200μA and 250μA. Generally, R3 can be selected as 5.11kΩ. In order to ensure the same accuracy of the two outputs, IR1/IR2=Pout1/Pout2, where Pout1 and Pout2 are 24V/400mA and 72V/100mA respectively. In this way, IR1=4IR2/3. After obtaining IR1 and IR2 from the above formula, the positions of R1 and R2 are obtained by the following formula: R1=Vout1/IR1=174kΩ, R2=Vout2/IR2=68kΩ.

3.2 Selecting Transformer and MOSFET

The transformer turns ratio is a function of the input-output voltage ratio and the duty cycle. When the duty cycle is 50%, the input is 12V, and the output is -72, a turns ratio of 1:6 is required. When the output is -24V, the turns ratio Np:Ns is 1:2, so the transformer turns ratio is 1:2:2:2.

The maximum output power of the entire circuit is the product of the maximum input power and the conversion efficiency (E). The conversion efficiency (E) is a function of resistance loss, transformer loss, MOSFET on-resistance loss, input and output capacitance, and switching loss. Generally, it can be assumed that the typical value of R is 80%. The maximum input power is a function of the current sensing resistor, input voltage, output voltage, inductance value, transformer turns ratio (Np:Ns), and switching operating frequency. The specific calculation formulas can be referred to the following formulas:

PIN(MAX)=VIND(Vcs/Pcs-VIND/(2FoscL)

D=NpVOUT/(NpVOUT+NsVIN)

At the maximum load current, IIN=VOUTIOUT(MAX)/VIN(MIN)E. In the formula, E is the conversion efficiency, select 80%, VOUT=24V IOUT(MAX)=400mA, VIN(MIN)=10.8V, the average input current is 1.11A, for a duty cycle of 52.5%, the average switch current is 2.114A, select the primary inductor fluctuation current △Ilo 40%, then the primary inductance is:

Lp=VIND/(△ILfOSC)

Among them, △IL=0.4×2.114=0.846A, fOSC=250kHz, so when Lp=27μH, the primary peak current is 2.5A.

Due to the limitation of the maximum output value (5V) of the internal LDO of MAX1856, the external power switch needs to select an N-channel NOSFET driven by logic level. When the input voltage is lower than 5V, an N-channel MOSFET with a VGS of 2.7V or lower can be selected. In addition, other characteristic indicators need to be considered, such as: gate charge QG, on-resistance RDS, maximum drain-source voltage VDS and minimum threshold voltage VTH. For IR's IRLL2705, its QG is 17nC (VGS=5V), so the gate drive current is:

IGATE=Qgfosc=8.5mA

Where: fosc is 500kHz

3.3 Selecting the current-sense resistor, input and output capacitors, and diodes

After the inductor peak current is determined, the current sensing resistor is determined by the following formula.

Rcs=Vcs/ILPEAK=85mV/ILPEAK=33mΩ

In order to prevent the current detection comparator from being interfered by noise, a 100Ω resistor should be connected between the RCS and CS pins, and a 1000pF filter capacitor should be connected between the CS pin and GND.

The rectifier diode should be a high-speed, fast-recovery Schottky diode, and the average current should meet the following requirements:

ID=IOUT（1+VOUT Np/(Ns VIN)）+△IL Np/(2Ns)

At the same time, the shutdown voltage must be greater than VOUT.

The equivalent series resistance (ESR) of the input and output capacitors should be low.

3.4 Vibration elimination circuit design

MAX1856 can use current sensing resistor to achieve current mode control. When the switch is turned on, MAX1856 has a floating period of 100ns to reduce noise interference. Then the secondary inductor and output diode capacitor will form an oscillation circuit and reflect back to the primary side and add it to the current sensing resistor. The time will exceed 100ns, thus causing noise interference. R4 and C3 in Figure 1 can quickly eliminate vibration. The time constant of the vibration elimination circuit must be less than 100ns, and R4 should meet 50ns/C3.

4 Power Supply Design for Wireless Terminals

Figure 2 shows a power supply circuit for a wireless public telephone, which can also be used as a power supply circuit for a wireless WLL terminal. If the power supply of the wireless terminal is a 12V solar cell, the output requirements are 24V/200mA for the first path, -48V/100mA for the second path, and 5V/500mA for the third path.