Flyback Transformer Design for MAX1856 SLIC Power Supplies - AN1166

The subscriber line interface circuit (SLIC) provides DC power, ringing and supervision functions for the "plain old telephone service" (POTS) where alerting a telephone is a prerequisite but a central ringer is generally not available. SLICs have also been used in Terminal Adapters (TA), Voice over Internet Protocol (VoIP) and Network Termination (NT) applications. This application note provides a detailed design procedure for the flyback transformer that can be used with MAX1856 in SLIC power supplies. The MAX1856 offers a low cost solution for generating subscriber line interface circuits (SLIC) power supplies. The SLIC provides DC power, ringing and supervision functions for the "plain old telephone service" (POTS) where alerting a telephone is a prerequisite but a central ringer is generally not available. SLICs which incorporate the ringer function, have been used in Terminal Adapters (TA), Voice over Internet Protocol (VoIP) and Network Termination (NT) applications. This document presents a sampling of power supply reference designs for use with typical SLICs in twochannel and four-channel POTS loops. Each application has specialized requirements depending on the number of channels serviced and loop length. The requirements for three typical applications are presented in Table 1. Table 1. Reference Solution Requirement Guide Parameter Application 1 Application 2 Application 3 Input Voltage 12V±10% 12V±10% 5V±10% Telephone Lines Serviced 4 2 2 Ring Battery Voltage VBAT1 -80V -80V -80V Ring Battery Voltage Regulation ±6.25% ±6.25% ±6.25% Talk Battery Voltage VBAT2 -24V -24V -24V Talk Battery Voltage Regulation ±10% ±10% ±10% Ring Battery DC Load Current 25mA 120mA 120mA Talk Battery DC Load Current 120mA 60mA 60mA A flyback topology is used to develop the required negative voltages. At power levels below 75 to 100 watts, the flyback topology results in minimum component count and lowest cost as compared to other topologies. Figure 1 shows a typical application circuit using the MAX1856 to generate the ringer and talk battery voltages. Figure 1. Dual negative output power supply. Two of the most important factors in the design of a flyback power supply are the controller and the transformer design. These reference designs use transformers, which were specifically designed for these applications. Familiarity with flyback transformer design is necessary in order to choose the correct transformer for a given application. This application note discusses details of flyback transformer design for MAX1856 controller-based circuits. Familiarity with basic flyback converter operation is assumed. The important functional blocks of the MAX1856 are first discussed. The requirements in Table 1 are then considered in conjunction with the MAX1856 to determine the transformer parameters. Three reference application circuits serve as examples to demonstrate the design techniques. Presenting the efficiency and cross-regulation data for these three circuits, in conclusion, complements the article. The MAX1856 Current Mode Controller The MAX1856 is a PWM peak current mode controller. The MAX1856 uses a direct summing configuration to process the output error signal, the current sense signal and a slope compensation ramp. The current sensing block and the slope compensation technique have a direct bearing on the application circuit and are discussed in greater detail in the following sections. Slope Compensation in The MAX1856 For slope compensation, the MAX1856 adds a fixed ramp generated by the oscillator to the current ramp. The slope compensation is therefore a function of both frequency and duty cycle. The current ramp signal is derived from the external switch current by sensing the voltage across the sense resistor (R1 in Figure 1) placed in series with the source of the external MOSFET. Stability requirements dictate that the added slope be at least equal to half the secondary current slope in magnitude. Increasing the added slope beyond the magnitude of the secondary current slope does little to further improve stability. The amount of added slope is fixed internally for a given duty cycle and frequency. The MAX1856 internal slope compensation circuit adds a linear ramp that increases from 8mV to 50mV over 90% of the switching time period for any given frequency. The offset of 8mV allows the MAX1856 controller to go to minimum duty cycle in the presence of light loads. The slope compensation influences the choice of the primary inductance of the flyback transformer. This will be discussed in greater detail in the section on "Transformer Design-Primary Inductance LP ". The MAX1856 Current Sensing with Blanking The MAX1856 senses the external switch current using a sense resistor R1 (Figure 1). Once the current across R1 exceeds 85mV the PWM ON time terminates. A leading edge spike is present on the current sense waveform when the external switch turns on. It arises from the following three causes: (1) Reverse recovery current in the rectifier diodes when the flyback operates in continuous conduction mode (CCM). If the MOSFET turns on during the reverse recovery, the diode acts as a short and a large current spike flows in the MOSFET. This spike is reflected in the current sense signal. Using an ultra-fast rectifier reduces the magnitude of this current. Higher leakage inductance in the transformer also reduces the magnitude of this spike. However, increasing the transformer leakage inductance is not a viable option since that will lead to large voltage spikes across the rectifier diode and across the MOSFET during turn-off. In addition, the secondary leakage inductance of the transformer forms a resonant circuit with the parasitic capacitance of the rectifier diode. This ringing is also reflected in the current sense waveform (R5 and C10 in Figure 1 form a snubber to critically damp this ringing). (2) The total capacitance on the MOSFET drain side includes the MOSFET drain-source capacitance (Cds), parasitic transformer winding capacitance, junction capacitance of the snubber diode (Cj) and any other trace capacitance on board. At turn-on, the total equivalent capacitance discharges through the MOSFET and current sense resistor. (3) The MOSFET gate operating current also flows through the current sense resistor. The MAX1856 deals with this leading edge noise by using blanking. Since the problem noise arises immediately after turn on, the MAX1856 ignores the current sense line for 100ns after turn on. As shown in Figure 1 an additional external low pass filter (R2, C7) may be used. The RC time constant of this filter should not be too large as that may distort the current sense signal. Typical values recommended are 100Ω for R2 and 1000pF for C7 (see the MAX1856 data sheet). The MAX1856 Switching Frequency The MAX1856 switching frequency can be varied from 100kHz to 500kHz based on the choice of resistor R4 in Figure 1. The switching frequency fsw is set by R4 as fsw = (5 × 1010)/R4.