[Design View / Design Solution]
Powering The Signal Path
Using an integrated flyback IC along with post filtering will deliver a high-performance split-rail supply.
Flyback Topology A flyback switching topology converts a wide input voltage into one or more output voltages with the option of isolating the output(s) from the inputs. As discussed earlier, output isolation is used when the power supply is delivering power to a remote location, where the ground reference voltages may differ. In place of the inductor found in other switching-regulator architectures, a transformer (or coupled inductors) is used as the inductive storage element.
The flyback topology employs a switch between the primary winding of the transformer and ground (Fig. 3). When the switch is closed, energy is transferred from the input to the primary winding. When the switch is opened, energy is moved from the secondary of the transformer to the outputs.
A PWM modulator controls the switch S1. The modulator is stimulated by an error amplifier, which creates an error term related to the output voltage in relation to a reference voltage. When the flyback control loop is in steady-state continuous-conduction mode, the PWM duty cycle (D) relates to the VIN, VOUT, and the turns ratio of the transformer and the forward-voltage drop of secondary catch diodes.
D = (VO + VF)/(VO + VF + ((NS / NP) x VIN))
Where:
NS = turns on the secondary windings NP = turns on the primary windings VF = forward voltage drop of the catch diodes
The modulator and loop error amplifier are internal to the flyback regulator IC. Some designers consider flyback regulators problematic and shy away from using them. By using recent integrated circuits such as National Semiconductor Corp.’s LM5001 (see “LM5001 Backgrounder”), and an available transformer, one can produce a relatively simple design and realize a superb power source for analog circuitry. As with any design, one does need to be aware of potential issues, including the inherent switching noise generated by a switching power circuit.
The LM5001 regulator toggles between two phases at the switching frequency (Fig. 4). During phase 1, the primary switch S1 is closed causing current flow into the transformer core (shown in blue); the secondary catch diodes (D+ and D–) remain off; and energy is delivered to the output from the output capacitors (C+ and C–). During phase 2, the secondary delivers energy to the outputs and output capacitors via the forward-biased catch diodes (shown in red). The output capacitors are charged during phase 2 to ensure continuous delivery of energy during phase 1.
Current changes paths during transitions between the two phases, causing voltage excursions that must be considered. As the regulator enters phase 2, S1 is opened and the primary current continues to flow as the field collapses. The result of this current flow will be a high voltage induced across the switch.
This transient voltage can result in two different problematic issues: the voltage may exceed the limits of the IC itself, and the high voltage transients can cause coupled and emitted noise. Placing a circuit across the primary winding of the transformer (often called a snubber) can limit this voltage, significantly reducing both of these issues. A simple RC and/or Zener diode is often used.
Snubber circuits can dramatically reduce voltage transients and noise, yet the designer must analyze the extra power dissipated in the snubber circuit and may need to trade off between noise and power efficiency. A snubber circuit may also be needed across the secondary catch diodes to limit voltage excursions when the diode turns off, although this may be a minor contribution to the noise budget.
No power supply is perfect. Yet when properly designed, a flyback regulator topology has some key advantages when developing split analog rails.
Key Design Elements One can design a physically small flyback circuit to deliver very low-noise rails for split-rail analog systems. The flyback circuit described below employs an LM5001 integrated flyback regulator with a very small surface-mount EP5 transformer. This circuit inputs 10 V to 30 V and comfortably delivers 250 mA on each output (±5 V). A system requiring higher current can use the LM5000 to deliver about twice this amount of power.
Isolation: When input to output isolation is needed, one also must add isolation from the output(s) back to the regulator’s feedback input. This can be accomplished by simply adding an LM431 (or LMV431) voltage-reference/amplifier IC to monitor the output voltage and feedback an error current through an optoisolator.
Primary and secondary ground planes should be sufficiently spaced yet connected with a high-voltage capacitor to reduce unwanted output noise during switching. Isolation limits are set by the breakdown limits of the transformer, the optoisolator, and the capacitor across the ground planes.
Transformer: The transformer should be selected properly for ideal operation, smallest size, and lowest cost. Transformer design involves theory that is not included in this article. Popular inductor companies are capable of using the key parameters from a design to recommend an available transformer or produce a transformer optimized for an application.
The key parameters for the transformer include physical size, inductance, and turns ratio. The physical size and type of core determines the amount of energy that can be stored (in phase 1) before saturation. A larger-sized core also provides the ability to lower the equivalent series resistance (ESR) by using lower gauge wire, resulting in lower losses. In general, the larger the transformer, the more power that can be delivered and/or the better the efficiency.
The inductance determines the transformer’s rate of storage and discharge and ultimately the magnitude of the transformer’s ripple currents. Higher inductance will lower ripple current and can improve operation. However, as inductance increases (more turns) the resistance of the winding increases, resulting in higher transformer losses. These tradeoffs should be considered when selecting a transformer.
As briefly discussed above, the turns ratio is determined by the range of VIN and VOUT. The limits of the turns ratio (NS/NP) is determined by the IC duty cycle limit and the range of VIN. I target a PWM duty cycle (D) between 25% and 45%. The minimum and maximum pulse width can easily be calculated at a specific operating frequency and compared to the limits specified for the IC being used. Reference the regulator’s datasheet for more details on setting the operating frequency.
Tradeoffs exist between size, inductance, and series resistance. All of these tradeoffs can be worked through by any application engineer working for one of the inductor companies.
Circuit protection: Current limiting and thermal limiting is provided by the LM5001 series of regulator ICs. The current limit is fixed as per datasheet specification. Though a direct secondary current limit isn’t available with basic flyback circuits, power delivered to a heavily loaded secondary will be limited by the IC primary switch current limit. If the secondary outputs are shorted, the regulator will limit current. Still, the circuit needs to absorb additional power, leading to increased temperatures on the transformer, the catch diodes, and the regulator IC.
Bench testing of a circuit (described later) shows that a shorted output (V+ to V–) results in increased temperatures on the IC, resulting in thermal shutdown of the IC (as per specification). After this fault condition, the shorted output must be removed and the input voltage cycled before the circuit will return to normal operation. As with any design, a thorough reliability study should be performed to ensure safe and reliable operation in the specific application.
Most system implementations work safely without extra protection circuitry on the secondary side of the transformer, yet if the power rails might be inadvertently shorted, some designers may opt to add a fuse or other means of extra protection.
As described earlier, during phase 1, S1 is conducting and energy flows from CIN into the primary of the transformer. S1 turns off at the end of phase 1, causing the primary current to continue to flow into the high impedance of the FET. The result is a voltage equal to IPRIMARY x ZFET_OFF. Without a snubber, the switch voltage could exceed beyond the maximum voltage allowed, resulting in damage to the IC.
An optional steering diode reduces the energy losses in the snubber by only providing current flow during the transition from phase 1 to phase 2, while remaining off during other phases of operation. The Zener diode (Z1) provides a worst-case clamp to ensure the switch voltage doesn’t exceed the maximum allowed by the IC. The Zener will only turn on under worst-case extremes, as when the outputs of the circuit are shorted and then opened up.
Switching Noise and Frequency Switching power supplies all exhibit noise associated with the charge and discharge cycle of the output capacitor (called ripple noise), as well as the transient noise caused by the turn on and turn off of the switching devices (discussed earlier). To reduce ripple noise, boost the switching frequency and/or increase the inductance of the transformer (Fig. 5).
Higher frequency reduces ripple noise, yet there’s always a limit to the switching frequency. Frequency limit is a function of the switches being employed (often inside the IC) and the pulse-width limit of the regulator itself. At higher frequencies, the output switches will dissipate more power caused by the ac losses. This loss should be considered when selecting a switching frequency.
Because the transformer turns ratio in a flyback converter can be selected to provide an ideal input/output voltage ratio, the minimum pulse-width limit is often not a limiting factor with a flyback design (as it is with other switching topologies). The flyback controller does have a maximum oscillator frequency that must be understood, yet switching losses often limit the frequency well before the limit of the oscillator itself.
In the design shown later, 600 kHz was chosen for the switching frequency based on the tradeoff of efficiency and size of the transformer. At 600 kHz, the design was able to obtain efficiencies above 80% while utilizing a very small EP5 transformer.
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