|Passive UHF RFID Tags|
|RF to DC|
|Getting Started, Getting Data|
|Tag IC Overall Design Challenges|
To operate, a tag IC needs not just power, but direct-current (DC) power: a source of voltage that is roughly constant in time, of magnitude from 1 to 3 V depending on the type of transistors used in the circuitry, and capable of supplying a few tens of microamps of current. The tag needs to get this DC power from an incoming RF signal whose polarity changes about 900 million times per second, and with the proviso that at a few meters from the reader, a small tag antenna provides an open-circuit voltage of only about 0.1–0.3 V.
To change alternating (AC) voltages to DC, we need an electrical component that treats positive and negative polarities differently: a diode. The left side of Figure 5.2 shows the idealized version of a diode: a component that allows electrical current to flow only in one direction. The right side of the figure shows a more realistic view of a diode’s characteristics: in the allowed (forward) direction current turns on slowly until some turn-on voltage is reached, thereafter increasing more rapidly. In the blocked (reverse) direction, a small leakage current flows, increasing as reverse voltage is increased.
The actual current flow through a diode is exponential in the voltage; a reasonable approximation is:
where I is current, V is voltage, q the charge on an electron, k is Boltzmann’s constant and T the absolute temperature. I0 is a constant characteristic of the particular type of diode in question. The quantity kT/q is about 0.026 V at room temperature, so if we increase the applied voltage by 1 V, we increase the current through the diode by:
That is, the forward current increases very rapidly indeed with voltage. For typical values of current flow, we can treat the current as turning on abruptly at some turn-on voltage Von. Two types of diodes are commonly available in standard IC processing: junction diodes, which have saturation currents I0 around 10−10 to 10−11/cm2 at room temperature, and Schottky diodes with saturation currents 3–7 orders of magnitude larger, corresponding to a reduction in voltage of about 0.2 to 0.3 V for the same current density. (Schottky diodes are more difficult to fabricate and are not always available, or may increase the processing cost when used.) Rectification can also be accomplished using diode-connected transistors, in which the drain is shorted to the gate; in this case, the turn-on voltage is roughly equal to the transistor threshold voltage.
This idealized version of the current-voltage characteristic is shown in Figure 5.3. The current is zero for all voltages less than the turn-on voltage Von, and can become arbitrarily large when the applied voltage exceeds the turn-on voltage. In this view, the diode acts as an ideal switch with an offset voltage. The offset voltage can be estimated from equation (5.1), and varies logarithmically (thus rather slowly) with the DC current required.
Armed with this simplified component model, let us examine the problem of extracting a DC voltage from the RF voltage provided by the antenna to the tag IC. The simplest approach is to place our idealized diode in series with the voltage from the antenna. The result ought to be current flow only in one direction through the diode. We’ll use a capacitor to store the current between RF cycles. (Recall that a capacitor is the analog of a spring. The voltage across the capacitor is proportional to the total amount of current that has flowed into it—the stored charge—analogous to the total extension or compression of a physical spring.) We will represent the remainder of the IC by a load resistor, through which current flows from the capacitor and diode. The whole scheme is shown in Figure 5.4.
Operation of this circuit, treating the diode as an idealized rectifier, is shown in Figure 5.5. When the voltage across the diode is larger than Von, the diode acts like a closed switch, with a voltage offset. A net voltage of (Vpk−Von) appears across the capacitor and resistor, where Vpk is the peak voltage of the signal. During this time, a pulse of current flows into the capacitor to charge it up.
When the voltage on the diode falls below Von, the diode turns off. Current now flows out of the capacitor through the resistor, and the voltage across the resistor decreases. The time required for the capacitor to discharge is equal to the product of the capacitance and resistance, RC. If this time is long compared to the RF cycle time, the supply voltage will be roughly constant during the RF cycle.
Let’s make a simple estimate of the component values required. The RF cycle time is about (1/900 MHz) = 1.1 nanoseconds. Let us assume that the IC consumes about 30 μW from a power supply of 1 V. Since the power dissipated in a resistor is proportional to the square of the voltage and inversely proportional to the resistance, we find
To achieve an RC time constant ten times longer than the RF cycle, we need:
This is a very modest amount of capacitance, even for an IC implementation. So far, it appears easy to convert incoming RF voltages to DC to power the circuit. However, it isn’t sufficient for the DC power to be constant over a single RF cycle: it is necessary that the tag still be powered even when the RF power is briefly switched off to send data to the tag. For plausible data rates, the power could be off for around 10 microseconds. To achieve this RC time constant, we need a capacitance of:
To achieve a reasonably constant supply voltage over the course of an RF cycle, we naturally need much more storage capacitance: on the order of 300 pF to keep the variation in supply voltage small. This is a substantial amount of capacitance and will require about 40 000 to 60 000 square microns of the IC (whose total area is typically 500 000 to 1 000 000 square microns). In addition to just storing enough charge, we also need to distribute the stored charge over the circuit so that those locations that need power at any given moment have it available; otherwise, one transistor switching on will tend to reduce the power supplied to its neighboring transistors, leading to crosstalk and logic errors.
In addition to the problem of providing enough capacitance, we need to provide the full operating voltage of the IC from the available RF voltage from the antenna. This is also challenging, because by reference to Figure 5.5, we can see that the output voltage of the simple rectifier is not the peak voltage of the input but the difference between the peak voltage and the turn-on voltage of the diode. If the incoming peak voltage is less than the diode turn-on voltage, the diode will never turn on and no power will be delivered to the circuit. Even when the incoming voltage is large enough to get through the diodes, the sacrifice of Von is painful: an IC needs 1 or 2 V to run, and the turn-on voltage of a typical diode at the relevant current densities might be around 0.5 V (rather dependent on how much diode area we are willing to devote). All this has to be squeezed out of an antenna that is itself providing only about 0.2 V at a distance of a few meters from the reader antenna. How are we to get enough voltage to run the chip?
The first tool we can make use of is reactive matching. The voltage provided by the antenna is associated with a specific source resistance and reactance; typically, the source resistance varies from a few tens to a few hundred ohms depending on the antenna configuration. The IC draws a few microamps at a volt or two, so the dissipative part of the IC appears as a rather larger resistance (typically 1–10 kΩ). By using inductors or capacitors to match the source and load, we can theoretically gain an increased voltage proportional to the square root of the ratio of these impedances. However, it is not practical to extend this approach indefinitely. Real matching elements have finite loss, and a very high Q also results in narrow bandwidth. For example, let us assume that wish a tag to operate over the whole region of frequencies in use worldwide, that is from 860 to 960 MHz. The relative bandwidth ought to be around (100/900) = 11%, so the matching network should have a quality factor around 1/0.11 = 9 or 10. Therefore, we can achieve an increase of 5- to 10-fold in the antenna voltage using reactive matching.
A very common approach to obtaining higher voltages from a rectifier is the use of a charge pump: a number of diodes connected in series so that the output voltage of the array is increased. The simplest sort of charge pump, a voltage doubler, is shown in Figure 5.6. Two diodes are connected in series, oriented so that forward current must flow from the ground potential to the positive terminal of the output voltage VDD. A capacitor prevents DC current from flowing between the antenna and the diodes, but stores charge and thus, permits highfrequency currents to flow. A second capacitor stores the resulting charge to smooth the output voltage.
When the RF voltage is negative and larger than the turn-on voltage, the first diode is on (Figure 5.7). Current flows from the ground node through the diode, causing charge to accumulate on the input capacitor. At the negative peak, the voltage across the capacitor is the difference between the negative peak voltage and the voltage on the top of the diode. The output (right) plate of the capacitor is (Vpk − Von) more positive than the RF input.
When the RF input becomes positive, the first diode turns off and the second (output) diode turns on (Figure 5.8). The charge that was collected on the input capacitor travels through the output diode to the output capacitor. The peak voltage that can be achieved is found by adding the voltage across the input capacitor, which we found above, to the peak positive RF voltage and subtracting the turn-on voltage of the output diode:
In the limit where the turn-on voltage can be ignored (e.g. when the input voltage is very large), the output DC voltage is double the peak voltage of the RF signal, from which fact the circuit derives its name. The actual output voltage depends on the amount of current drawn out of the storage capacitor during each cycle, that is on the value of the load resistance (not shown here).
To produce higher output voltages, we can provide additional stages of multiplication to produce a Dickson charge pump. A two-stage configuration is shown in Figure 5.9; in the case of ideal diodes with negligible turn-on voltage, the output would be four times larger than the peak RF input voltage. In general, for N stages we find:
It is tempting to imagine that one could continue to add as many stages as required to convert even the most modest input voltage into something adequate to power the IC, but as we add stages, we encounter diminishing returns. A very simple analysis of the problem is shown in Figure 5.10. All the DC current must flow through all the diodes in series, so as we add more stages, we waste more and more power in the turn-on voltage of the diodes:
The power efficiency of the charge pump thus decreases as the number of stages increases for a given turn-on voltage and output voltage:
(This analysis turns out to be a bit optimistic for the single- and two-stage cases if substrate loss—current flowing into the bulk of the silicon wafer due to the capacitance of the diode to the substrate—is significant.)
The resulting behavior is shown in Figure 5.11. We can see that the more stages we add (to enable the IC to run with a smaller RF power and thus extend the nominal range), the less efficient the charge pump becomes.
A reasonable approach to estimating the number of stages is to extract an equivalent resistance from the load, given the total power calculated above:
where the input voltage is adjusted to produce the requisite load voltage from equation (5.7). Roughly speaking, the largest resistance that can be matched to the antenna is Q2 times larger than the radiation resistance of the antenna. For a typical dipole-type antenna, this value is 10–50 Ω, so the largest equivalent resistance that can be optimally matched is around 5 kΩ, assuming the limits on matching mentioned above. (Higher values can be used but at some sacrifice in bandwidth.) We can thus, adjust the number of stages in the charge pump to provide about the right equivalent resistance for the value of Q we expect to achieve in matching.
Several weaker but non-negligible effects are important in arriving at a final design. The area of the diodes has a weak (logarithmic) effect on the turn-on voltage and thus on the efficiency, so one is tempted to make the diodes large. However, the diode capacitance grows linearly with the diode area, and since the equivalent resistance of the charge pump is fixed (by the power and voltage targets, as described above), as the diodes are made larger the capacitor starts to draw a substantial reactive current. The capacitance is also voltage dependent (increasing noticeably as the diodes are turned on), and the variation in capacitance degrades the performance of the matching network, particularly for narrow band, high-Q networks. A charge pump with more stages has smaller capacitance variations because the peak voltage across each diode is closer to the turn-on voltage; a typical change for a junction diode is around 25–30% of the zero-bias capacitance. Larger diodes contribute more capacitance but need less peak forward voltage for the same current, so the capacitance variation grows rather more slowly than the capacitance itself. A plausible guideline is that the change in reactance of the equivalent input capacitance be comparable to the equivalent resistance of the load, leading to an input capacitance of around 0.25–0.5 pF for typical parameter values. The exact results are, of course, sensitive to the details of the process technology used.
Even from this rough modeling approach, it is clear that key leverage in operating at higher efficiencies and lower power lies in reducing the turn-on voltage of the diodes comprising the charge pump (ideally without excessive increases in diode capacitance), and it can be expected that progress along those lines will continue to improve passive chip performance.