Don’t get caught up in the concept, the simplest thermal loop analysis text is here

When it comes to switching regulators and their electromagnetic compatibility (EMC), hot loops are always mentioned. Especially when optimizing the trace layout on the printed circuit board, it is even more inseparable from this topic. But what exactly is a thermal loop?

When it comes to switching regulators and their electromagnetic compatibility (EMC), hot loops are always mentioned. Especially when optimizing the trace layout on the printed circuit board, it is even more inseparable from this topic. But what exactly is a thermal loop?

In a switching regulator, current needs to be switched constantly. These currents are usually relatively large. Whenever current flows, a magnetic field is created. If a large current is switched quickly, an alternating magnetic field is created. Additionally, if there is parasitic inductance in the path when switching currents, voltage offsets can occur. Currents can capacitively couple into adjacent circuit components and increase noise radiation from the power supply. To sum up, we can say that switching current is the main cause of noise in switch-mode power supplies. Figure 1 shows a simplified buck converter topology. All lines with continuous current are shown in blue. All lines that quickly switch current are shown in red.

Figure 1. Lines with continuous current are shown in blue, and lines with switching current are shown in red.

The red line in Figure 1 is the critical line. They look like a current loop, hence the name loop. A hot loop means that this loop is especially critical because it involves switching currents quickly. If we look closely at this loop, we can see that the red loop in Figure 1 never really flows, because the two switches never turn on at the same time. It’s just a combination of individual lines, with current flowing at certain times and no current at other times. In FIG. 2, arrows beside each connection line indicate the direction of current flow. Meanwhile, the time during which the current flows is represented by a designated symbol. At other times, no current flows through the conductor.

Table 1 shows when each red wire in Figure 2 conducts current and when it does not. During the turn-on period of the buck regulator’s duty cycle, the high-side switch is on and the low-side switch is off, and we see current flowing from the input capacitor through the high-side switch, but no current flowing through the low-side switch. During the off period of the duty cycle, current flows through the low-side switch (from ground to the switch node), and no current flows through the other three red lines.

Table 1. Relationship Between High-Side and Low-Side Switching States and Buck Regulator Duty Cycle Period

It is easy to see from Figure 2 that the hot loop is not an independent current loop, but a virtual current loop composed of two real current loops.

Figure 2. Individual lines in a hot loop with different current directions.

Figure 3 shows the actual current loop on which the circuit is based. One current loop is shown in blue and the other is shown in green. Between these complete current loops, switching does occur repeatedly; however, in some circuits, the current flows in the same direction in both current loops, and thus superimpose to form a continuous current, which has little effect in terms of EMC. These lines will not be called hot loops.

Figure 3. Actual current loops that lead to so-called hot loops.

The hot loop of a switching regulator varies depending on the topology of the switching regulator. Its design should be as narrow and compact as possible to reduce noise generation and transmission. Silent Switcher from Analog Devices® 2 technology minimizes critical thermal loops by integrating the input capacitance into the IC package. At the same time, dividing the thermal loop into two symmetrical shapes can generate two magnetic fields with opposite polarities, which can substantially cancel the radiated noise. The LT8609S from ADI Power is a switching regulator using this technology

・ Silent Switcher® 2 Architecture:

§ Ultra-low EMI/EMC emissions on any PCB
§ Eliminates PCB layout sensitivity
§ Internal bypass capacitors reduce radiated EMI
§ Wide input voltage range: 3.0V to 42V

・ Ultra-low quiescent current Burst Mode® operation:

§ § Output Ripple
・ High Efficiency 2MHz Synchronous Operation:

§ >93% Efficiency (at 1A, 5VOUT from 12VIN)

・ 2A maximum continuous output, 3A peak transient output
・ Fast minimum turn-on time: 45ns
・ Adjustable and synchronizable frequency range: 200kHz to 2.2MHz
・ Allows the use of small inductors
・ Low dropout
・ Peak current mode operation
・ Internal compensation
・ Output soft start and tracking
・ Small outline 16-pin 3mm x 3mm LQFN package