MAXREFDES1090

Due to its simplicity and low cost, the flyback converter is the preferred choice for low-to-medium isolated DC-DC power-conversion applications. However, the use of an optocoupler or an auxiliary winding on the flyback transformer for voltage feedback across the isolation barrier increases the number of components and design complexity. The MAX17690 eliminates the need for an optocoupler or auxiliary transformer winding and achieves ±5% output voltage regulation over line, load, and temperature variations.

The MAX17690 implements an innovative algorithm to accurately determine the output voltage by sensing the reflected voltage across the primary winding during the flyback time interval. By sampling and regulating this reflected voltage when the secondary current is close to zero, the effects of secondary-side DC losses in the transformer winding, the PCB tracks, and the rectifying diode on output voltage regulation can be minimized.

The MAX17690 also compensates for the negative temperature coefficient of the rectifying diode.

Other features include the following:

• 4.5V to 60V Input Voltage Range
• Programmable Switching Frequency from 50kHz to 250kHz
• Programmable Input Enable/UVLO Feature
• Programmable Input Overvoltage Protection
• Adjustable Soft-Start
• 2A/4A Peak Source/Sink Gate Drive Capability
• Hiccup Mode Short-Circuit Protection
• Fast Cycle-by-Cycle Peak Current Limit
• Thermal Shutdown Protection
• Space-Saving, 16-Pin, 3mm x 3mm TQFN Package
• -40°C to +125°C Operating Temperature Range

An isolated no-opto flyback DC-DC converter using the MAX17690 is demonstrated for a 12V DC output application. The power supply delivers up to 250mA at 12V. Table 1 shows an overview of the design specification.

This document describes the hardware shown in Figure 1. It provides a detailed systematic technical guide to designing an isolated no-opto flyback DC-DC converter using Analog Devices MAX17690 controller. The power supply has been built and tested.

One of the drawbacks encountered in most isolated DC-DC converter topologies is that information relating to the output voltage on the isolated secondary side of the transformer must be communicated back to the primary side to maintain output voltage regulation. In a regular isolated flyback converter, this is normally achieved using an optocoupler feedback circuit or an additional auxiliary winding on the flyback transformer. Optocoupler feedback circuits reduce overall power-supply efficiency, and the extra components increase the cost and physical size of the power supply. In addition, optocoupler feedback circuits are difficult to design reliably due to their limited bandwidth, nonlinearity, high CTR variation, and aging effects. Feedback circuits employing auxiliary transformer windings also exhibit deficiencies. Using an extra winding adds to the flyback transformer’s complexity, physical size, and cost, while load regulation and dynamic response are often poor.

The MAX17690 is a peak current-mode controller designed specifically to eliminate the need for optocoupler or auxiliary transformer winding feedback in the traditional isolated flyback topology, therefore reducing size, cost, and design complexity. It derives information about the isolated output voltage by examining the voltage on the primary-side winding of the flyback transformer.

Other than this uniquely innovative method for regulating the output voltage, the no-opto isolated flyback converter using the MAX17690 follows the same general design process as a traditional flyback converter. To understand the operation and benefits of the no-opto flyback converter it is useful to review the schematic and typical waveforms of the traditional flyback converter (using the MAX17595), shown in Figure 2.

The simplified schematic in Figure 2 illustrates how information about the output voltage is obtained across the isolation barrier in traditional isolated flyback converters. The optocoupler feedback mechanism requires at least 10 components including an optocoupler and a shunt regulator, in addition to a primary-side bias voltage, V_BIAS, to drive the photo-transistor. The error voltage FB2 connects to the FB pin of the flyback controller.

The transformer feedback method requires an additional winding on the primary side of the flyback transformer, a diode, a capacitor, and two resistors to generate a voltage proportional to the output voltage. This voltage is compared to an internal reference in a traditional flyback controller to generate the error voltage.

By including additional innovative features internally in the MAX17690 no-opto flyback controller, Analog Devices has enabled power-supply designers to eliminate the additional components, board area, complexity, and cost associated with both the optocoupler and transformer feedback methods. Figure 3 illustrates a simplified schematic and typical waveforms for an isolated no-opto flyback DC-DC converter using the MAX17690.

By comparing Figure 3 with Figure 2, it is evident that there is no difference in the voltage and current waveforms in the traditional and no-opto flyback topologies. The difference is in the control method used to maintain V_OUT at its target value over the required load, line, and temperature range. The MAX17690 achieves this with minimum components by forcing the voltage V_FLYBACK during the conduction period of DFR to be precisely the voltage required to maintain a constant V_OUT. When Q_P turns off, DFR conducts and the drain voltage of Q_P, rises to a voltage V_FLYBACK above V_IN. After initial ringing due to transformer leakage inductance and the junction capacitance of DFR and output capacitance of Q_P, the voltage V_FLYBACK is given by:

where:

V_FLYBACK is the Q_P drain voltage relative to primary ground V_DFR(T) is the forward voltage drop of DFR, which has a negative temperature coefficient

I_LS(t) is the instantaneous secondary transformer current R_S(T) is the total DC resistance of the secondary circuit, which has a positive temperature coefficient

n_SP is the secondary to primary turns ratio of the flyback transformer

The voltage of interest is (V_FLYBACK - V_IN) since this is a measure of V_OUT. An internal voltage to current amplifier generates a current proportional to (V_FLYBACK - V_IN). This current then flows through R_SET to generate a ground referenced voltage, V_SET, proportional to (V_FLYBACK - V_IN). This requires that:

Combining this equation with the previous equation for V_FLYBACK, we have:

We need to consider the effect of the temperature dependence of V_DFR and the time dependence of I_LS on the control system. If V_FLYBACK is sampled at a time when I_LS is very close to zero, then the term I_LS(t) x R_S(T) is negligible and can be assumed to be zero in the previous expression. This is the case when the flyback converter is operating in, or close to, discontinuous conduction mode. It is very important to sample the V_FLYBACK voltage before the secondary current reaches zero since there is a very large oscillation on V_FLYBACK due to the resonance between the primary magnetizing inductance of the flyback transformer and the output capacitance of Q_P as soon as the current reaches zero in the secondary, as shown in Figures 2 and 3. The time at which V_FLYBACK is sampled is set by resistor R_VCM.

The V_DFR term has a significant negative temperature coefficient that must be compensated to ensure acceptable output voltage regulation over the required temperature range. This is achieved by internally connecting a positive temperature coefficient current source to the V_SET pin. The current is set by resistor R_TC connected to ground. The simplest way to understand the temperature compensation mechanism is to think about what needs to happen in the control system when temperature increases. In an uncompensated system, as the temperature increases, V_DFR decreases due to its negative temperature coefficient. Since V_DFR decreases, V_OUT increases by the same amount, therefore V_FLYBACK remains unchanged. Since V_SET is proportional to V_FLYBACK, V_SET also remains unchanged. Since there is no change in V_SET there is no change in duty cycle demand to bring V_OUT back down to its target value. What needs to happen in the temperature compensated case is, when V_OUT increases due to the negative temperature coefficient of V_DFR, V_SET needs to increase by an amount just sufficient to bring V_OUT back to its target value. This is achieved by designing V_SET with a positive temperature coefficient. Expressed mathematically as:

where:

δV_DFR/δT is the diodes forward temperature coefficient

δV_TC/δT = 1.85mV/°C

V_TC = 0.55V is the voltage at the TC pin at +25°C

Rearranging the above expression gives:

The effect of adding the positive temperature coefficient current, TC, to the current in R_FB is equivalent to adding a positive temperature coefficient voltage in series with V_DFR on the secondary side of value:

Substituting from the previous expression, this becomes:

Now substituting this expression into the expression for V_OUT gives:

and finally solving for R_FB:

Values for R_SET, V_SET, and δV_TC/δT can be obtained from the MAX17690 data sheet as follows:

Values for V_DFR and δV_DFR/δT can be obtained from the output diode data sheet, and n_SP is calculated when the flyback transformer is designed.

The value of R_TC can then be calculated using the expression from earlier, restated below:

The calculated resistor values for R_FB and R_TC should always be verified experimentally and adjusted, if necessary, to achieve optimum performance over the required temperature range. Note that the reference design described in this document has only been verified at room temperature. Finally, the internal temperature compensation circuitry requires a current proportional to V_IN. R_RIN should be chosen as approximately:

Setting the V_FLYBACK Sampling Instant

The MAX17690 generates an internal voltage proportional to the on-time volt-second product. This enables the device to determine the correct sampling instant for V_FLYBACK during the Q_P off-time. The R_VCM resistor is used to scale this internal voltage to an acceptable internal voltage limit in the device.