[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.
Filtering Switching Noise Adding a noise filter to the output of a switching power supply is a good method of reducing switcher noise (Fig. 6). A simple LC low-pass filter can be employed to reduce ripple and transient noise to a level acceptable for most analog power supplies. This design adds a series inductor followed by capacitors to ground to provide the filtering needed. One can use various combinations of inductors and capacitors to do the job.
In the application described below, I started with a low value inductor, which provided a low ESR to minimize IR droop and power losses across the inductor (Fig. 7). I targeted a 40-db noise attenuation at an octave below the 600-kHz operating frequency, so I used a 60-kHz, 3-db point for my calculations. A 1-µH inductor value results in a capacitor value of 7 µF, so I used a small 10-µF ceramic capacitor that I had in my lab, C = 1/(f2 x 4π2L). If desired, you can also start with a capacitor value and calculate for the inductor value.
In lieu of an LC filter, some designers may opt for an additional linear regulator after a switch-mode regulator. The idea is to use a switching regulator to regulate to just above the voltage needed for the linear regulator to operate properly (called the dropout voltage). Driving the linear regulator input just above the dropout voltage creates an accurate voltage while minimizing energy losses.
Linear regulator circuits can be used as noise filter circuits, but be aware that a linear regulator itself doesn’t provide much switching noise attenuation. Linear regulators usually have minimal effect on noise above just a few kilohertz, yet properly chosen input and output capacitors can provide significant low-pass filtering. For very sensitive analog circuitry, pay attention to the noise generated by the linear regulator. Low-noise linear regulators are available for biasing of such circuitry (see the specific regulator datasheet for details).
Based on the additional efficiency loss and circuit cost, designers often find a passive filter acceptable for filtering switching noise for most low-power analog rails. For high-power analog rail generation, the IR drop across a filter inductor may result in unacceptable droop or load transient regulation, justifying the use of an additional active element.
Stability All power supplies that employ feedback should be checked for stability over all operating conditions. As with any amplifier, a gain of greater than 1 with a phase shift of 180° results in oscillation. Most power supplies are compensated with a simple RC delay circuit placed across the error amplifier. The idea is to attenuate the gain of the amplifier at higher frequencies, where the phase delay approaches 180°. Because the loop bandwidth is limited, the circuit’s ability to react to fast load changes is also limited.
With fast-changing digital circuits, an over-compensated regulator circuit may inhibit the regulator’s ability to change load current fast enough, resulting in voltage droop, overshoot, and ringing as load currents change. For most analog rails, the load current is fairly constant, so this is usually not a problem. As a result, the loop bandwidth can be limited without any significant effect.
It’s a good idea with any design to verify the circuit’s stability over operating conditions. The loop stability can be checked using different methods. One of the simplest methods is to load-step the output current from its minimum to maximum over the operating conditions of the design (VIN range, temp, etc.).
During these load-steps, observe the output voltage to ensure the voltage(s) remains within tolerance and returns to the proper level without any significant ringing, overshoot, or undershoot. A good rule of thumb is to ensure that the voltage returns to a stable output within approximately three ring cycles, and the excursions aren’t outside of system limits. If excessive ringing is evident, adjustment of the compensations circuit may be necessary.
For those that have access to a network analyzer, one can verify operating extremes and directly read phase margin off of a Bode plot. More information concerning measurement of phase noise and stability can be found at www.national.com/an/AN/AN-1889.pdf.
Circuit Performance Measurements The circuit in Figure 7 and Figure 12 is a working design that produces +5 V and -5 V rails on a small double-sided PCB. These power rails are appropriate for use in biasing most sensitive analog circuits. This circuit has been successfully employed in designs where the power is delivered from a remote power source (10 V to 30 V) that’s a significant distance away from this circuit. The outputs are fully isolated from the input, so the earth grounds at the two locations can vary without affecting the circuit being powered.
This design provides clean and stable rails for powering a remote analog (and/or digital) system with superior system performance. Other low-noise split-rail power designs often require multiple regulators, resulting in higher noise, larger PCB size, and higher cost. For a more detailed explanation of the circuit operation and the full design package (PCB, BOM, etc.) please go to www.national.com/rd/RDhtml/RD-171.html.
Output regulation: Not all analog systems that employ split-rail biasing are affected by variations of the actual rail voltages. Signals are often centered by ac coupling and biasing between V+ and V–, thus as long as the differential rails remain high enough in amplitude for the signal to remain within an acceptable common-mode range, all will operate properly. In any case, proper regulation must be considered and in this case the target was 5% of the nominal rail voltage.
A flyback design must take into account a few points when it comes to accurate regulation. Because the feedback control loop only comes off of the positive output, the outputs must maintain a small load at all times to ensure 5% regulation (30 mA with this design). If the load current is less, the negative output voltage may float below –5.25 V.
Also, the +5 V rail should carry a small load to ensure that the negative rail maintains proper regulation during high negative loads. If the positive output doesn’t carry this small load, when the negative output is loaded, the regulator’s on time may be reduced to a point to where the transformer core doesn’t receive sufficient energy to maintain the loaded negative output.
Because most designs will require a minimum current well above these low limits, no extra design effort is needed. For designs with very low shutdown currents (< 30 mA), a load resistor can be used to ensure the voltage stays within tolerance.
Regulation of the positive rail stays within 5% tolerance over loads from 25 mA to 250 mA (Tables 1 and 2). The negative rail is slightly less accurate, yet also maintains 5% cross regulation with loads above 30 mA. Though the negative rail isn’t directly regulated with a feedback loop to the regulator, it provides acceptable regulation via mutual coupling of the transformer.
Output noise: In Figure 8, noise was measured on each rail with respect to the output ground and the differential +5 V to –5 V noise. In all cases, the transient noise was below 20 mV p-p and ripple noise below 5 mV p-p. Because of the symmetry of the flyback secondary design, some differential noise may cancel. Symmetrical layout of the secondary circuit and optimized transformer design improves differential noise cancellation.
The noise measurements were taken using a high-performance differential probe with ac coupling placed directly across the output connector P2. Be aware that some of the rounding of the triangle wave caused by the ripple voltage results from the ac-coupling capacitors within the probe used, and isn’t an effect of the circuit itself. The actual noise should be less than shown here.
Stability: Control-loop compensation is accomplished by the RC circuit, which is connected to the the LM5001’s COMP pin (R6/C13 and R9/C12). In this circuit, the feedback signal is safely fed into the same COMP pin of the regulator and the feedback (FB) pin is shorted to ground. This provides a method to bypass the voltage reference and error amplifier internal to the LM5001, allowing the use of a separate reference and amplifier on the secondary side of the transformer (U2 – LM431).
The design shown is very stable and provides over 45° of phase margin. Because the output filter is outside the feedback loop, it doesn’t affect the stability of the control loop (Fig. 9).
Efficiency: Efficiency of the flyback design shown was measured to be above 80% over most of the operating range (Fig. 10). Power losses arise from the transformer, the catch diodes, and the IC itself (internal switch and biasing). As mentioned above, the switching frequency was limited to minimize the ac losses, yet still provide the advantages of a small transformer core size.
At low loads and high input voltages, IC biasing dominates the losses. At higher loads, the transformer begins to saturate and starts to dominate the loss budget. The design as shown runs without any single element losing significant amounts of energy, thus no component runs at elevated temperatures when delivering full power.
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