7.7 Appendix G: Power Supply Design Considerations
This development board was designed as an experimental platform for users interested in exploring the capabilities and the development procedures used for designing with PIC16F176x/177x CIP Hybrid Power Controllers. Thus, the fairly conventional synchronous buck converter topology has been enhanced with additional options and features to meet this key objective. However, some of the added features may not be ideal for a real-world design. Some of the limitations introduced by these trade-offs are described below.
Current sensing
- Current Sense Transformer CT
- Inductor DC Resistance Sensing Circuit DCR
- High-Side Shunt Amplifier HS
While DCR sensing is transparent and does not influence the operation of the buck converter, the high-side shunt resistor and the current sense transformer feedback circuits introduce the following limitations:
Current sense transformer feedback
The current sense transformers are very common in mid- and high-voltage designs where isolated feedback paths are required, such as power factor correction stages in AC/DC offline converters. To allow users to evaluate the tuning capabilities of specific IC features like leading edge blanking and to establish stable feedback loops by incorporating chopped feedback waveforms, a current sense transformer was also added to the CIP Hybrid Power Starter Kit. Although this feedback circuit is working well and reliably, it introduces a dominant noise source in this specific buck converter design, which results in dominant spikes in the conducted emissions spectrum (see the test results in Appendix F), and therefore may not be considered as a template for a real-world design of a buck converter.
High-Side Shunt Resistor Feedback
In addition to the CT feedback source, a high-side shunt resistor R1 of 10 mΩ was placed between L1 and the output capacitors C3, C4, C5, C7 and C42. This shunt resistor has significant influence on the efficiency of the power supply (see the test results in Efficiency Measurements figure in Appendix C) and slightly deforms the output voltage ripple due to the changing forward voltage drop across the shunt resistor dependent on the load current. In conjunction with the MCP6C02-020 high-side shunt amplifier having a fixed gain of 20, this current sense signal has an equal current feedback gain as the current sense transformer circuit of 5 A/V, to allow users to seamlessly switch between both signal sources, without having to change any additional feedback component on the board.
The high-side shunt current sense signal is routed to an alternate peak current feedback input pin where additional PIC16F1779 device configurations can be evaluated accounting for the phase-shifted, lower bandwidth signal waveform.
The high-side shunt current feedback signal is also routed to a separated error amplifier input, where it can be compensated using a type II or type III RC compensation network. By changing the PIC16F1779 device configuration accordingly, this average current feedback will be:
- Put in series with an outer voltage feedback loop to form an inner average current loop of a constant voltage source (common battery charger control system);
- Used independently as single average current control loop in a constant current source;
- Used as outer average current loop in conjunction with an inner peak current loop in a constant current source (common LED driver control system).
DCR Sense Feedback
Using inductor DC resistance sensing for generating an inductor peak current feedback signal is common practice in low-voltage, high-current designs. This current sensing options is popular as it does not directly affect the operation of the converter and also does not introduce additional sources for noise or losses like other sensing techniques. However, the signal size depends on the DC resistance of the inductor. Thus, high-current, high-efficiency DCR designs often suffer from poor signal noise ratio and, as the signal is produced by an RC filter only, also is often not very linear and distorted. Yet again, the highly flexible peripherals of PIC16F1779 allow users to experiment with various options to account for these shortcomings and help to optimize the overall performance of systems using DCR sensing as inductor current feedback.
Furthermore, as the DCR sensing method is also very cost effective by incorporating only a very small number of passive components, an on-chip amplifier is used to condition the feedback signal, showcasing further ways of how power supply designs can be optimized by utilizing PIC16F1779 features.
Input Filter
The synchronous buck converter is equipped with a small PI filter at its input to reduce incoming noise injected by attached DC sources. However, the filter design may only be partly sufficient to suppress conducted noise generated by the CIP Hybrid Power Starter Kit itself, depending on the cables and connections used. One of the major noise sources is the current sense transformer circuit described above. For this design, this trade-off was solved in advantage of the usability and value for the experimental evaluation process rather than providing a real-world design template meeting emission standards.
Main Inductor L1 Footprint
The inductor L1 component solder pads available on the CIP Hybrid Power Starter Kit have been shaped to allow users exchanging the main inductor against different components for experimental purposes. These pads support footprints for SMT inductors within the same power range of major inductor vendors.
The main inductor populated by default is composed of a compound material incorporating iron-powder and ferrite materials in a specific ratio to achieve low inductance derating overload current up to the nominal current rating while still having a relatively soft saturation characteristic at the same time. The DC-resistance was selected to be in a range of 5-10 mΩ for an optimized DCR sensing feedback signal size.
Output Bulk Capacitor C2 Footprint
The output bulk capacitor C2 solder pads available on the CIP Hybrid Power Starter Kit have been shaped to allow users exchanging the output bulk capacitor against different components for experimental purposes. These pads support footprints for SMT capacitors of major capacitor vendors.
High-Speed Switch Node
The half-bridge switch node of this synchronous buck converter is composed of a high-side MOSFET MCP87050 with less gate charge (Qg) but slightly higher on-resistance (RDS(on)) and a low-side MOSFET MCP87022 with higher gate charge (Qg) and lower on-resistance (RDS(on)) to account for the shared power losses in the nominal operating conditions at approx. 25-35% duty ratio. Microchip’s MCP87xxx family of Power MOSFETs are high-speed Si-MOSFETs with rise and fall times with less than 5 ns when driven by an appropriate high-speed FET driver like MCP14700.
See the AN1471 - Efficiency Analysis of a Synchronous Buck Converter using Microsoft Office Excel-Based Loss Calculator application note for further information.
In the CIP Hybrid Power Starter Kit the switching edges were slowed down with the penalty of higher switching losses to account for the negative impact of the parasitic inductance introduced by the primary winding of the current sense transformer. This inductance results in a voltage overshoot when the high-side switch is closed. This switching spike is also clearly visible in the conducted emissions test results.
Although using a low-inductance current sense transformer and test results are still within a reasonable range to meet EN55022 Class B, it is recommended to take special care of a short and tight loop with minimum parasitic inductance between the input capacitors and half-bridge MOSFETs to achieve optimum results when migrating this switch node to custom designs.