Patient safety is a global health priority. Recalling resolution WHA55.18 (2002), which urged Member States to “pay the closest possible attention to the problem of patient safety and to establish and strengthen science-based systems, necessary for improving patients’ safety and the quality of health care”, the seventy-second World Health Assembly (WHA72), in May 2019, adopted WHA72.6, a resolution on ‘Global action on patient safety’. (Source: https://www.who.int/patientsafety/en/)

Multiparameter Patient Monitors measure human vital signs like Electrocardiogram (ECG), Blood Oxygen Concentration (% SpO2), blood pressure, temperature, and so forth(see Figure 1-1).

Figure 1-1 Multi-Parameter Patient Monitor Showing How Modules are Connected to Main Monitor

Figure 1-2 shows a generic block diagram of the ECG module. It highlights the isolation between the analog circuit and the digital processing unit. Medical standards (like IEC60601-1) specify that multiparameter patient monitors need to have isolation up to 5kV to separate the grounds, prevent any ground loop currents, and prevent leakage currents flowing from the patient’s body. While isolating data, it also needs the same level of isolation as on the power supply.

Figure 1-2 Isolated ECG Module Block Diagram (showing isolation)

The data isolation can be done using simple digital isolators but the isolated power topology needs a thoughtful analysis. Table 1-1 shows the typical specs for such isolated power supplies.

This application report dwells deeper into critical design challenges associated with isolated power and data such as output regulation, feedback mechanism, input voltage range, output power and size considerations along with suitable power architectures given below:

• Conventional and PSR Flyback – Wide input voltage (Vin) and adjustable output power (Pout)
• Open-loop and Closed-loop Push-Pull – Feedback mechanism and output regulation
• Integrated isolated power and data – form-factor constraint designs with limited Pout

A typical conventional Flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. It can generate voltages below or above the input voltage and can support output power levels easily up to 10s of watts (depending upon the transformer design). To get the isolated output information back to the Flyback controller (aka PWM controller) device in order to accurately regulate the output, the output is fed back using either an opto-coupler based circuit or a tertiary winding as shown in Figure 2-1. In such cases, the component count and the solution size is fairly large. In fact, reliability can be a concern with opto-coupler based feedback and an additional winding (for tertiary winding based feedback) adds to the cost of the transformer.

It is important to properly design the Flyback transformer for best performance. The transformer should be very well coupled with low leakage inductance for highest efficiency and best regulation (especially in multiple outputs). The parasitic capacitance from the primary to secondary must also be limited to prevent excessive Electro-magnetic Interference (EMI).

Figure 2-1 Conventional Flyback Feedback Mechanism (a) Using Opto-Coupler (b) Using Tertiary Winding

The PSR Flyback topology eliminates the need of an opto-coupler or tertiary winding and provides extremely tight load regulation using the feature of the resonant ring caused by the magnetizing inductance and the switch node capacitance. Compared to conventional Flyback, it operates in Discontinuous Conduction Mode (DCM) or Boundary conduction Mode (BCM) depending on the load. It, in turn, eliminates any errors from the transformer DC resistance (DCR) or secondary side diode to provide tight regulation.

The PSR Flyback operates in BCM/DCM with lower inductance (given the higher peak-to-peak ripple current) and lower switching losses compared to Continuous Conduction Mode (CCM) because the MOSFET turn-on and diode turn-off are at zero current. However, higher primary and secondary-side RMS currents in PSR Flyback result in conduction losses in the magnetic and semiconductors.

Figure 3-1 shows an example schematic for PSR Flyback topology using LM5180 device. It shows how transformer has only two windings and overall number of components is also less compared to conventional Flyback.

Figure 3-1 PSR Flyback Configuration Showing Very Less Number of Total Components and Single Transformer